Start out by laying out the PCB traces & pads with your favorite CAD or PCB design program. I used Pad2Pad, mainly because I found that program pretty easy to use, and it's free for download
. You're supposed to use Pad2Pad to design a board, then send the file to the company for manufacture. Instead, I use this design to create my own etching mask.
Unfortunately, you can't export p2p files into other formats. So, I printed the board layout to a PDF, then opened the PDF into Illustrator, which allowed me to clean up & fine-tune the design and separate it into layers. This technique is for creating single-sided PCBs, so I made two masks: One for the copper traces & pads, the other for the silkscreen. You need to print out a mirror image of your masks -- you'll see why soon -- but because the traces go on the back of the board, you can print those out normally. The silkscreen mask should be printed out in reverse.
I use 2pt lines for most of the traces; that comes out to about 0.028".