I started with PCB design first. I went off the deep end with a couple of designs Bolle for vintage boards that Bolle had captured the schematic for, and designed a new PCB in the form factor I wanted. Not exactly for the faint of heart...
I recommend spending a little time learning an EDA first - Kicad being the most common choice. Once you have the hang of capturing schematics in there, you need to create/add symbols for the major components (ICs, connectors, etc) and then painstakingly capture every connection on the card, typically done by toning it out with a multimeter. If you have the experience, you can make certain assumptions to speed things up, for example, +12v on one PDS connector is probably connected to other +12v pins and you might not need to tone that unless you have reason to believe otherwise.
Repeat until you've got all the connections verified on the card. Then you need to capture the contents of the PLDs (U1 probably is a GAL/PAL), and start thinking about designing a PCB. It's a bit of a process.
