Making A Solder Mask Template
The first thing I need to do was to make a solder mask template. This is basically a transparent film that has artwork of the pads and vias in your PCB design.
DIY Sharpie Solder Mask
If your PCB design is simple, you could DIY your own solder mask template with a sharpie. Overlay a piece of transparency film over the PCB and mark in the pads and vias using a fine tipped sharpie. You'll want to double up on transparency film to make the pads more darker, which will be helpful to block out the UV light when exposing the solder mask resin.
For more complex designs, I suggest making a custom template with the help of your CAD software. In Autodesk Eagle, it's as easy as isolating the pads and vias layer. I also suggest showing the mounting holes and board outline, which is helpful for obtaining good alignment. You can use the "Print" feature to save out a PDF.
You may find it useful to tweak the pads and outlines using a vector graphics editing software (such as Adobe Illustrator or Inkscape) – I found it necessary to fill in the holes in the pads so the resin doesn't harden when exposed. You can also add vector graphics for getting DIY silkscreen graphics.
You'll want to layout multiple copies of the PCB so that they can be doubled up. I did this by copying and pasting the vectors in Adobe Illustrator and tried to maintain even padding/margin space between them.
Printing on Transparency Film
Getting the solder mask artwork onto a transparency film can be done with either laser toner printer or an inkjet printer. The inkjet printers are more common and require a special "Quick-Dry" type transparency paper. One side has a coarse texture which is the side we’ll need to print on. The rough surface allows the ink to dry so it sticks and it doesn’t wipe off.
Using the print dialog, I changed the paper type to the glossy paper and cranked up to the highest quality print setting – This will ensure the ink sticks to the transparency film.
Using an x-acto knife, cut out the artwork and make sure to leave padding around the edges so it’s easier to handle the film. You'll need to double up the artwork because the ink just isn’t dark enough to fully block out the UV light.
To stick them together, I added drops of superglue along the edges. You’ll want to make sure not to get this on the ink, so make sure there’s plenty of blank space. Perfect alignment is very important, so you may want to tack the artwork down so it stays in place. Blank pieces film can be used to spread the resin, so make sure you have a few.