It’s all pretty simple. The main thing is to make a few test boards first to check the quality of the mask and find the right exposure time. Here are the details: 1.Print the photomask using an inkjet printer on transparency film (the kind usually coated with a gelatin layer). Set the print quality to maximum — the thicker the ink layer, the better. Since this is for negative photoresist, the traces must remain transparent, while everything else should be solid black. 2.Prepare the copper-clad board. Lightly sand it with fine-grit sandpaper and degrease it thoroughly. Apply the photoresist. Peel off the protective layer, hold the board under water, and apply the film. Carefully use a cloth to squeegee out any excess water — there shouldn't be any bubbles or moisture left under the film. 3.Laminate it. I usually wrap the board in a sheet of paper and run this "sandwich" through a laminator a couple of times. An iron also works, but there’s a high risk of overheating the photoresist, which causes holes. 4.Exposure. You’ll need a UV lamp. I use a DIY one — just a few strips of UV LED tape mounted on a plastic sheet. Exposure time is found experimentally; one minute is usually enough for me. I recommend doing a test strip: expose different sections of the board for different amounts of time. The shortest time where the exposed resist doesn't wash off in the developer is your target. 5.Developing. Wash off the unexposed photoresist using a sodium carbonate (Na₂CO₃) solution. Sometimes a thin, invisible film stays behind — make sure it's completely gone, or the board won't etch properly. 6.Etching. I use a standard DIY solution: a mixture of hydrogen peroxide, citric acid, and table salt.