Sarahpink, I presume you have Garmin "Training Center" on your computer, if so:-
Download your ride into it.
Look in the history (in Training Center) for the ride.
Right click on it, and create a course from it.
Go into course's (in Training Center) and find the course that you have just created.
Edit the course and you can put in all your Turns, hills etc wherever you want them.
Once done, just upload to the Garmin
I think you can do it in Bikeroute toaster by loading it with a GPX and it probably will do it automatically and be an easier way, but I have no experience of it as I avoid having instructions on a course as I find it can slow the Edge down updating especcially on very long rides and it becomes easy to miss your tun.
Alternativey create your route from new in Bikeroute toaster and if you hit the right buttons before you upload it will also add the turn instructions
The Edge I use is the 305, so other models may have a faster processor and not have the same problem.