If you were to hex edit a QVM and make it work, I think you would instantly surpass most of the mod creators :>
Best thing to do is probably to grab a mod patch - find a mod that mostly fits what you want, get the patches needed to create it. From those you should also get what version of the source was used to generate the patch, so you can use subversion to download that version of the source. Apply the patch to the source, and at that point if you were to compile it you'd have the exact mod that you got the source for. Now you can edit to your heart's content; change things around, see what breaks, etc. Build the QVM, load it up in devmap in your client and see how it works for you. Lather, rinse, repeat.
I would go into more specifics about the process, but I'm running on about three hours of sleep right now and if any of the above makes sense at all I'll be surprised. Instead, I'll point you at what is probably the most used QVM out there,
Lakitu7's. Combine that with some of the threads that already exist about compiling Tremulous and you should be on your way. 'man' pages are always your friend if you get lost, as are Google, and if it's specific to Tremulous itself then of course here as well.