With the 305, there's a difference between courses and routes. A course is intended more for a known ride and contains extra stuff like the elevation profile and pace data. It'll bleep at you if you wander off course and bleep again when you're back on course but that's about it.

With a route, you can set actual waypoints (up to 100 per route I believe) and the unit will alert you as you approach the next waypoint in your route and give a distance remaining countdown. It's just a simple matter of placing your waypoints at the different junctions along your route and using a naming convention like "LT", "RT", etc. and you'll get very usable navigation.

I use Garmin's own Mapsource program to create the routes and upload them directly to the unit. It's most useful if you own a Garmin map product otherwise you're limited to the very sparse basemap which can make more complex route planning difficult. But even without a detailed map you can still use it to import .gpx files made with other online route planning products and write them to the unit.

There's a tutorial at http://frank.kinlan.co.uk/?page_id=410 that explains the process in a bit more detail. I don't mess with the whole GPSbabel part it covers opting to use Mapsource instead.