Building a modular synth without some way to make printed circuit boards is almost impossible. When i say almost impossible it's because i have seen a few people who have actually build entire synth's on Veroboard or perf board.
It's possible but not for the faint hearted - like me.
When you need a PCB there are to different approaches, have a (hopefully) professional PCB house do the work for you or du it your self.
If you want someone else to do it for you you need the PCB layout in a special format (usually as Gerber files but others exist), then you just send the files to the PCB house and they send you the boards usually drilled and often with a solder mask indicating which components go where.
I suffer from a combination of a bad case of "not invented here" and a limited budget so i naturally have to do my own boards.
I use the "photo" approach to making PCB's. I could use the pre coated photo sensitive boards, but they are expensive and come only in a limited number of sizes. So i make my own photo sensitive boards. To do that you need :
Raw cobber clad board cut to the size needed for whatever you are building (ad a little extra for mounting the board)
A special photo sensitive spray, i use a German brand called "positiv 20" from a company called kontakt chemie.
A UV light source, first i used one of those cheap halogen work lamps, but it's not really an UV lamp and exposure times were very long, so i build my own UV exposure box out of a Philips facial solarium rehoused in an old defect scanner. I bought the solarium at a flea market for 55 Danish ( less than 10 US $), the scanner was free. 2 hours of work and now i have a perfectly working UV exposure box. Only limitations is that it can not do two sided boards (neither can i :-) ) and theres no build in timer. I got the idea to use the solarium from Jørgen Bergfors site, and the idea to build it in to an old scanner from this site.
You also need to develop the boards. You can get special purpose developers, but they are expensive and do not last long. I use 7 grams of caustic soda in one litre of water, this develops many boards, and is very very cheap.
Finally you need to etch the boards, i use good old ferric chloride.
So the whole process goes like this.
First i print the PCB layout on ordinary white paper, but on a good laser printer. I found that color laser printers tend to make black "more black" than ordinary laserprinters. I tried using ink jet printers but the print is simply not black enough, and the edges are often blurred. So i stick with the color laserprinter at work.
Next step i to cut a suitable piece of raw cobberclad board to the needed size + an inch extra at one end for the brace that i use to mount the board to the frontpanel. I sand the edges of the board (remember dustmask). The cobber site of the board needs to be very clean, so i use the finest steel wool i can get and "scrub" the board (in one direction only) until it's perfectly shiny. Then i clean it with rubbing alcohol. After this i don't touch the cobber until the board is finished.
Then i coat the board with photo sensitive lacquer, the trick here is to get an even layer, not too thick. I dry the board in an oven at 70 degrees centigrade for 15 minutes (as stated on the lacquer bottle).
The printed PCB design is cut to size and placed on the glassplate of the UV exposure box, and the coated and dried board is placed on top of that, coated cobber site down of course. Remember not to get the PCB deign mirrored the wrong way when you print it. Exposure time is about 30 seconds.
After exposure the board is immediately placed in the developer. Development takes about 2 minutes, depending on how much photo lacquer was applied. It's important that all the excess photo lacquer is removed in the development process.
After development the board is rinsed under cold running water and etched in ferric chloride. I like to speed the etching up by putting the ferric chloride in a hot water bath. After etching another rinse in cold water.
Finally the photo lacquer that's still left on the remaining cobber is stripped of with rubbing alcohol.
Now all that's left is the tedious job of drilling the board. I use my drill press for that, again remember a dust mask. I tried drilling boards with my Dremmel tool but i always end up breaking a lot of drill bits when i do that. A drillpress (mine is just a cheap one) and good quality carbide drills is the best way i have found.