Before you begin, I recommend thinking a little more carefully about what you want to do. Just like there are many synths to buy, there are many you could build. You can definitely go for some of the many DIY projects out there, but I’d recommend considering what you want to build and why; hardware or software, analogue or digital, monosynth, drum synth etc? Additionally what type of format you’d like to target: standalone, semi modular, modular? For me, for example, eurorack gave me a format in which I could make something piecemeal out of simpler parts. It also let me more easily experiment with individual elements of a synth Vs designing something all in one. I could make a simple voice, a custom trig sequencer, unusual modulation sources etc. Personally, modular was a great driver for improving my analogue circuit design skills (which was intersting in itself) too.