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The tricky part is getting the two layers lined up. How hard that is depends on how small your traces and holes are. This is how I do it...
( I will assume you are basically familiar with single-side PCB making, and want to do prototype-level work with minimal equipment, like me. If you have access to professional stuff like spray etchers and mask printers, lucky you )
Equipment: Pre-sensitized double-sided PCB, light box, UV light box, 1.6mm-thick card, cellotape, small paper clips (dog-clips). A cheap, low-power vertical micrscope helps.
1. Draw on the artwork (if you are making it on a PC) alignment lines - horizontal and vertical lines at the edges.
1. Make the artwork. Make sure that the printed side will go towards the copper, otherwise you will get poor exposure.
2. On a light box (or bright paper), lay the two images over wach other. The printed sides should be in the middle.
3. Align the alignment lines. You should be able to see light though all the hole pads. A microscope, fixed vertically, helps here as it looks straight down (no parallax error) and increases detail.
4. Clip the two images together at one edge. Check alignment again.
5. Insert a slim (5-10mm) length of card into the opposite edge, between the images. The card edge should go up to where the PCB will be. Clip this edge together.
6. Remove clips at other side. Insert card pieces on remaining threee edges and clip the opposite edge again.
7. Check alignment again carefully. Here, a microscope really helps to eliminate parallax error.
8. Tape the two unlipped edges together with the card in place (it helps to leave a few mm of card sticking out from the images for the tape to stick to).
9. Remove clips and tape one of the remaining edges with the card.
10. The fourth piece of card is removed. You are left with a 'pocket' that you can insert the PCB into. The PCB should be a tight fit into this to prevent movement, especially if you only have a single-sided UV box - as you will have to turn over the PCB, moving things if it is loose!
11. With the PCB in place, expose to UV. Be very careful if you have to turn over the PCB to expose the other side.
12. Etch by your preferred method.
That's it... at last! Care and patience are of paramount importance. Do it right and you will do it just once to get a good board. Rush, and you will likely have to start all over again. All the equipment can be improvised.
One thing that would make it a little easier, try to find a transparant piece of plastic the same thickness as your PCB and cut it to the same size. This can be placed between the cardboard strips as you make the 'pocket', making alignment and constuction much easier. I haven't got around to this bit yet.
You might also want to check this post, about making images, laser printer choice, etc.
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