You buy a bottle of liquid photoresist and coat the board in dim light. Used to be known as KPR- Kodak Photo Resist. I believe board houses apply a film of some sort. No idea what you can still buy. I gave up on all that nonsense, and just print my layouts on a laser printer, then iron them onto the board with high heat. Soaking in water allows the paper to be rubbed off, leaving the plastic toner adhered to the board. Done with care, quite fine lines can be achieved. Fastest process I've used. Next project I'll just send CAD files to AP Circuits and wait a couple days for finished boards to arrive on my doorstep. Getting lazy in my old age 
Get a good brand of photoresist spray.... like Positiv 20...
normaly it needs to dry for half a day or so in a dark place....to ensure good attachment with copper.
I put mine in the oven, turn it up till the pilot light shows \normaly around 60 to 70C), set timer for 10 minutes.... at 10 minutes swich off, at 15 minutes remove and cool in dark place for an hour or 2... the second method gives me most consistant results.... you can also give the board a final light coating on missed spots, when you take it out of the oven, the heat will dry that last coat quickly.. I hang a cloth over the oven's handle to keep light out, but you don't have to worry too much about light at this stage... just keep things reasonably dim.
The second method has added benifit of quicker drying... less time for dust to settle....
Biggest secret is cleaning the PCB first... use lots of acetone.... I lay out kitchen towels and then drop a bit of 'tone on the board (at an angle) and try to cover as much area with the liquid as I can before it reaches the bottom... you will see the dirt float off...even after you thought it was clean.... melamine counter tops should be fine with acetone. rubber gloves also wont hurt. its annoying how far oil will creep from your skin, when you are just holding the edges....
normaly it needs to dry for half a day or so in a dark place....to ensure good attachment with copper.
I put mine in the oven, turn it up till the pilot light shows \normaly around 60 to 70C), set timer for 10 minutes.... at 10 minutes swich off, at 15 minutes remove and cool in dark place for an hour or 2... the second method gives me most consistant results.... you can also give the board a final light coating on missed spots, when you take it out of the oven, the heat will dry that last coat quickly.. I hang a cloth over the oven's handle to keep light out, but you don't have to worry too much about light at this stage... just keep things reasonably dim.
The second method has added benifit of quicker drying... less time for dust to settle....
Biggest secret is cleaning the PCB first... use lots of acetone.... I lay out kitchen towels and then drop a bit of 'tone on the board (at an angle) and try to cover as much area with the liquid as I can before it reaches the bottom... you will see the dirt float off...even after you thought it was clean.... melamine counter tops should be fine with acetone. rubber gloves also wont hurt. its annoying how far oil will creep from your skin, when you are just holding the edges....
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