Gear First off for basic navigation most recreational\navigation grade GPS units like a Garmin Etrex are just fine. Some of the higher end navigation grade units like the Garmin 60CSX collect points better under heavy tree canopy and get a fix faster. One good reason to get a unit with a lot of memory is that you can now put as a back drop large portions of 1:24,000 quad maps. It used to be that you only could put 1:100,000 scale topos which did not offer a lot of detail but might save your bacon showing you the nearest highway or major road. In order to see much of a 1:24,000 scale map it also helps to have a bigger screen. For further reading on the different grades of GPS units here is a good site. http://www.forestpal.com/GPS.html There too many different makes and models to be covered here but most will do the job for navigation. Delorme now makes a unit in conjunction with the Spot Satellite service allows you to send text messages. So now you can twitter in the back-country. It might be a bit difficult to text and mountain bike at the same time although I am sure some will try.Seriously though this might be a good feature to be able to communicate with if you were in trouble.
Turn it on and get a fix Before you start riding, the unit has to get a fix on at least three satellites for latitude and longitude and four to get altitude. This may take several minutes or more. Typically the unit will open to a default screen showing the satellites it is seeing and trying to get information from. This will vary every time. Some days the satellite positioning maybe really good and others not. If you are in a tall forest or on a steep hillside it may be difficult for the unit to see satellites. If you are really desperate if you wait long enough you may get better satellite positioning later. See the discussion below on how to hold the unit. Once it is ready to go there should be a message to that effect and usually a guess as to how accurate the reading is plus or minus a certain number of feet. Without getting in to a technical discussion on the reasons why, all GPS units are considerably less accurate in vertical accuracy (elevation) than in horizontal accuracy, usually about two or three times less accurate. Some higher end gps units may have a barometric altimeter built in that would be more accurate than anything the GPS signals could provide.
Collecting Trails and Points Most units will have a icon for collecting tracks and a button or icon for marking points. It would be a good idea to first practice by marking a point like the trail head or your car. When you press the button or icon usually called "Mark" it will give you some information about your current location and offer a name. Most likely you will want to change the name to something meaningful. You can't screw that up because the unit will only let you enter certain things. Just say OK after naming it and it should be available to point to later if you need it.
To collect a track there will typically be a menu item by that name. You should start by clearing out any previous data related to tracks because otherwise it will connect the dots. There will be a line from where ever the unit was collecting track information last and where you are now. You can fix that with software later but it is much easier on the gps unit now. This is a good habit to get in to for new tracks. After you clear it there should be some screen choices for stopping and starting data collection like on and off. If you need to stop and fix a flat or fertilize the woods you should turn the track log because otherwise it will keep collecting points and you will have a whole jumble of them. Just remember to turn the track log back on when you get started again! When you are done save the track and give it a name you will remember. There is an exception to this advice. If you are loading a track onto something like Open Street Map you will want to not save it and instead import the active log file because it will have the time the points were acquired. This time stamp information may also be useful if you are associating time stamped pictures with gps points. Also in some units, mostly older ones, the line may be simplified when saved to make the file smaller so there will be less data points fleshing out the line. You can save the file and then use the log files stored on the unit for time stamps but that does not qualify as GPS basics.
Yet another issue to be aware of is that many units like Garmins have a feature that snaps your current location to road nearby if you are close enough. The page listed below says to turn that feature off by "To disable this on the Garmin eTrex Legend, use the 'Page' button to select the 'Map' page. Use the thumb stick to select the options menu at the top of the page. Within the menu select 'Setup Map'. Within the sub-menu select 'Lock on Road' and set it to 'Off"
Here is a similar page to this one on GPS basics that is well done http://mtbtrailguide.com/php/instructionshtml.php
Entering or Downloading to the GPS The are a couple of basic ways to enter in known coordinates or tracks. By the way a route is just way points along a trail and not enough coordinates to make a precise representation of a trail. If your map has coordinates listed for important features you can enter them in manually as long as they are in the same units (See the discussion on Units and Datum below) One other way to enter in gps data to your gps is with gpx files. A gpx file is a universal standard that all modern gps devices should be able to use. Your gps device should have come with some sort of software that will allow you to make this transfer. Another real easy way to transfer data is if you have a program like National Geographic Topo! state series where you can find your area of interest as a digital quad map. You then mark points on the map and or trace trails and then transfer them on to your gps. The tracks you trace will be transferred as routes typically.
Another way to get coordinates for manual entry is by using Google Earth. When you add a placemark and move it where you want it the coordinates will be listed. Also you can get coordinates in Google Maps by right clicking and picking get directions from here and then moving the marker off the road.
How to hold the unit. Depending on the antenna of the unit some get the best signal if they are held flat and others if they are held upright. Garmin Etrex and Gekos work best flat. but some other Garmins like the 60csx work best upright. Check your unit online to see which is the best way to position it. Most units will take the abuse of a handlebar mount if they are available. If you are collecting points or are in a real bind to get a fix in an emergency, face the unit to the southern sky for better accuracy. If you need to get a signal but can't, try to move away from tall trees or cliffs. The more unobstructed view of the sky you have the better. If you hold the unit close to your body it could interfere with getting signals from some satellites. Some units have optional external antennas that you can mount on your helmet for better reception.
Datum The datum used on a map and set on a GPS unit refers to a theoretical mathematical representation of smoothed earth. Your GPS unit probably comes set to WGS84 which is the same one used in applications like Google Earth. Unless you have a real good reason to change it leave it at that. Most modern mountain bike maps will use a datum like NAD83 if they are showing GPS grids or location coordinates on a map. (NAD stands for North American Datum) The difference between NAD83 and WGS84 is insignificant for purposes of navigation. One instance where you might want to change the datum is if you are using an old USGS 7.5 minute quad map. They are usually in NAD27. The difference between NAD27 and NAD83 is enough to matter for navigation so you should probably change it if you are using a quad map.
Units Latitude and Longitude that the gps unit works with can be displayed in a few different ways. Typically the most common options are Degrees Minutes and Seconds or Decimal Degrees. The explanation of the math of how they relate is interesting but requires some study to fully grasp. For purposes of navigation you need to set the unit to whatever the map is using. To give you a hint about how it works here is an example. Longitude in Decimal Degrees might look like -121.5 but the same thing in Degrees Minutes and Seconds would be -121degrees 30(minutes) 0(seconds) It has to to with the number of degrees in a circle and the earth is a circle and not a square unless of course your religion says it is. http://id.mind.net/~zona/mmts/trigonometryRealms/degMinSec/degMinSec.htm
Most GPS units and mapping software allow you to change the units and datum very easily. This should not be something that is buried too deeply in the menu. It is a simple matter to enter the point in one datum and type of units and then switch to another datum and units. For example lets say say you create a point in Google Earth which is probably in WGS 84 and the units are in decimal degrees (to change those go to tools then options) Change your GPS unit settings to match that and enter the latitude and longitude. Now say you are planning on using a 7.5 min paper topo quad to navigate with. You would likely change the GPS settings to NAD 27 and the units to degrees minutes and seconds.
Navigating with the GPS If you want to know the distance and direction of a particular point you saved as a waypoint with the mark button or whatever it happens to be called on your GPS there will likely be a corresponding find button or a waypoint menu item. Keep in mind that unlike a car type GPS with roads built in your gps is only going to be able to tell you the distance "as the crow flies" and likewise the direction will be pointing directly to the point, not say which trail to take.
One of the oldest ways to use a gps to navigate is also the most difficult and that is to use a grid on a map to both figure out where you are based on gps coordinates or figure out where you want to go based on coordinates. This would be a good skill to learn hands on in an adult ed class. An important thing to get right here is the datum which will be listed on the map.
Here is a way to cheat a bit on this. If you know the coordinates of where you want to go and your gps says it is, for example, 1.5 miles. use the scale bar on the map to figure out what this length would be. If one inch equals a mile then take some straight edge (break a pine needle off at 1.5 inches?) put one end on the known location like a trail intersection then turn the measuring device until it intersects the trail you are on. That is where you are. This works really well for river running. The crow says it is 4 miles but it is more like 7 miles on the river. All this requires that you enter in the coordinates before you go into the field which is far easier if you have a digital topo program or use Google Earth\Maps etc.
Here are some resources on using a paper map grids and gps.
Below is a map set on this site with gpx waypoints that you can download on to your gps that correspond to the map. If you live in the Central Oregon area this might be a good one to practice with. There is no GPS grid on this map however. This would be for practicing entering information before you go in the field.