Use tracing paper. I'm having good results with fairly light paper, 75gsm. This paper will crinkle as it comes out of the laser printer; just leave it for a few hours pressed between the pages of a fat book, and it should flatten out nicely. Tracing paper also takes up the toner very well, giving a nice black image.
You'll be taking the design and placing it ink side down on the PCB (so that light can't sneak around the edges). This means that for a 2-sided PCB, the component side (the "top") will need to be printed "flopped", while the solder side (the "bottom") won't. Print the two sheets then hold them together, ink-side in the middle, to visualize what you're going to get.
Tape the two sides together at one edge, making sure that they are exactly registered. The board will go in the middle.
The manufacturers instructions don't talk about this. My first attempt was to pre-score the edges of the small board with a craft knife. This didn't work and scratched off little chips of copper from the surface. It's much better just to saw the edge with a hacksaw. Do this someplace other than the area where you'll make the exposure or the fiberglass dust will get everywhere.
The edge will be rough. If it sticks up from the plane of the board, it will prevent the design from lying flat on the copper, and cause a blurred image. File down all the edges with a nail file so they're smooth to the touch.
Make sure that the exposure area is clean. Stray dust on the exposure sheet will work just like black ink and prevent copper from being etched in that location.
Throw away the plastic sheet that comes with the exposure lamp, it flexes which makes it useless. You want something that will hold the paper in contact with the board, or the pattern will be out of focus. Glass is good. I took the glass from a couple of 4" x 6" picture frames ($0.25 from the thrift shop). Even the cheapest glass is really, really flat.
Sandwich the board and paper between two sheets of glass, with
strong clips holding the whole thing together.
Because tracing paper is quite opaque, the exposure time needs to be longer than that recommended in the manufacturers instructions, which seem to assume that you'll be using transparency film. I cut some small (1/2" square) pieces from a board and did some test exposures. 11 minutes exposure with 30 seconds development seems to give good results.
I use a cheap X10 controller to time the exposure. Using the bottlerocket program on Linux, I can use a line like this
br a1 on ; sleep 660 ; br a1 offto give an exact 11 minute exposure while I'm away doing something else.
For 2-sided PCBs, make seperate exposures for the 2 sides.
There's not much I can add here that isn't in the manufacturer's instructions.
Developing is the crucial part - the exposed copper should be bright and shiny, and the green resist should be thick and opaque. Brushing away the resist seems to help keep the development process even across the board, but be careful not to scratch the resist off.
I found that by filling the big plastic tray with boiling water, and putting the etchant in a small bucket floating in it, the etchant gets nice and hot pretty quickly. This gives an etch time of about 3 minutes.
Eventually I decided to stop doing dangerous chemistry in the kitchen and bought a heated etching tank ($37) and a couple of gallons of etchant from Ocean State Electronics. It works great, but it takes about 2 hours to get up to 50 degrees C.