Not hard to do - you have to remove the pin the holds the hook in the bell to do it (you drop the hook, place the spring around it, then re-install the hook). *Careful* with this - it *is* spring-loaded - but it's not much trouble. I did mine in about 10 or 15 minutes. You do need to have any drawbar or implement removed from the snap coupler bell to do it, though (an obvious point, I know, but some folks might try the short cut and get hurt).
You probably won't really need a diagram unless your snap coupler is currently disassembled. The spring is still available from AGCO if you don't already have it.