I have the Nuvi 550 - which is pretty much the same as the 500. I bought mine primarily for use in the water on my Waverunner, but it also gets use in my vehicle. Not so much on the bike however, I have a Garmin bike specific computer with the cadence/HRM and all that stuff. I like all that stuff, even if I really do not "need" it.
Anyway, it is fairly simple to add waypoints and create a route.
1. Enter bicycle mode. All this really does is create navigation routes that keep you off as many major roads as possible (when you ask the GPS to get you from point A to point B it will not select a route that takes you on an interstate highway for example). But you have a bicycle mode so why not use it.
2. From the main screen, select tools then custom routes
3. Say yes to create a new route
4. select "add new start point". Then add a start point using any of the icons (address, home, points of interest, recently found, favorites, intersection, etc). The most useful way to build a route is by selecting "browse map" or "coordinates", so lets just assume you will be using "select map".
5. The map will pop up on the screen. Touch the location of where you wish to start. Then press "select".
6. You now see a screen that shows the waypoints selected for the custom route. right now there should only be three things there, "add new start point", the point you just selected, and "add new end point". Your route contains one waypoint at this location.
7. At this point, there are two options. Each does the same thing. You have a start point, so now you need an end point. Select "add new end point". Now is where the two options come into play.
Option A is to select the nearest desired waypoint as your "end point". Lets say that is an intersection a block away. Select it. You will now be back to the screen that shows all your waypoints. You will see two selections. The start point is on the top, the end point is on the bottom. The red square deletes a waypoint, the green arrow inserts a new waypoint. Ignore the green arrow for now.
Your "end point" is now close to your start point, so simply add a new end point. Select "add new end point" and using the browse map feature, select your next waypoint. This becomes your new end point. Now you have three waypoints loaded, the start, the first waypoint, and the end. Keep adding a new end point, one intersection at a time. Then build your desired route. You do not need to add every intersection you pass through, typically you only add the turn points.
Option A builds your route from the start to the finish, with each waypoint in order.
Option B pretty much does the same thing, only you select your start point and then the ACTUAL end point. You use the green arrows to "wedge" in a new waypoint. So you have the start and the end. You select the green arrow that points between the start and end, then insert a waypoint. Now there are three waypoints, and two green arrows. "wedge" in another waypoint, then keep going till your route is complete.
Option B builds your route by selecting the start and end points first, then filling in all the details one at a time. I find this to be a little more confusing, so I would just go by option A.
There is free software you can get that works with a bunch of different GPS units called "Easy GPS". What it does is allow you to use your computer to create routes, then upload or download them to your GPS unit. For GPS units that are not touch screen this software really helps. For touch screen GPS units, it still speeds up route creation and waypoint entering if you use coordinates. You can also remove (and save) waypoints and routes off your GPS to reduce clutter - then when you want them back just hook the GPS up to the computer and upload them. You get Easy GPS at www.easygps.com
The best way to go about this is to try to create a simple route just in your neighborhood. Start off with something like a 4 point route around your block. Then start making larger routes. Try using the "add new end point" and also use the green arrows. See what happens to the route each time. You will get the hang of it.