I would be remiss if I didn’t follow mention of a burgeoning community in the real world for synth DIYers with the foremost virtual community:
You’ll find profiles of all the members of the Synth-DIY list. There’s quite a range of work, from assembled modular systems to circuit-bent and modified instruments to entirely scratch-built instruments. Several people here even modify their Ensoniq and Casio keyboards. (Sadly, they don’t fully pimp their synths, which clearly needs to be next.) So the good news is “DIY” can mean whatever you’d like it to.
Most importantly, you’ll find important reference material: schematics of basic components, plus a reference guide to electronic topics like soldering/desoldering, an unusually comprehensive look at resistors, and for the more ambitious, how to trim your resistors to a different value.
Pictured: Toby Pronteon working in his lab, not because this Seattle dorkbot regular has an extensive set of projects on his site like a Theremin-style controller with CV out, but mostly because this is what I imagine synth labs should look like. (Always wear a lab coat.)
Now, a lot of this stuff is decidedly retro in flavor. I wonder if a new generation of synth builders will give their gear a more futuristic digital/analog hybrid flair.