Your lucky day... bare with as I am doing this from memory.
On opening Mapsource, you should see the Track as a grey line on the map. If it's not there, select Tracks, Show on map and it should appear.
If the line is very dark, it means you have it highlighted. Un-click it and it should fade.
Now select the Route Tool button.
Click on the road at the start of the Track and follow it along. The magenta line should appear, tracing over the route.
Take a bit of care. Tracks are not 100% accurate and may not follow the roads exactly or, if they go down some smaller roads, the scale might need to be zoomed in a bit. Similarly, if the track runs down say a dual carriageway or motorway it's sometimes possible to click onto the opposite carriageway. It's easy to spot that happening as:
(1) If you click off the road, you should see a Waypoint appear in the Waypoints column. Correct the mistake using the undo button and zoom in.
(2) if you hit the wrong carriageway, you should see the magenta line coming back on itself. Again, click undo and all should be well.
Take a bit of time and you should get the hang of it really quickly. Zoom in and out a bit to pick up any glitches that might have appeared and the job's done.
Tracing is also useful if you have any old routes created in earlier map versions. Tracing over them in the new maps will convert them to match. It saves the GPS device doing it, which is where errors creep in. It also lets you take advantage of any new roads that might have appeared over the years, by-passes around towns for example.
When finished, I always delete the track from the file. Why? Tracks are often full of Waypoints that then clog up your Favourites. The second reason? You don't need it any more. I also delete all the Waypoints to stop them clogging up my favourites. Deleting them also acts as a second check that you have clicked on a road, not into a field. If you have you will see a prompt asking you if you really want to delete it? Nine times out of ten it's safe to say yes, Mapsource will make the logical assumption that you really do not want to go into a field but would rather go down the road. On the one time it doesn't you can undo the change using the back button and sort yourself out. Practice makes perfect, as usual.
I am using the Tracing method to create the six routes that will make up the Diagonal Across France routes in another thread from last night, using the .gpx files downloaded from the Moto magazine's website. I could use Basecamp's ability to turn a track into a route (or use my Garmin device to do it) but I prefer the slightly slower but ultimately more accurate (and pleasing, at least to me) old fashioned method. Not least as it lets me see what the route does, where it goes and what else might be around.
In short, play around with it. You can't break it. If you think you will, make a copy first. Come back if you have any questions.