The idea is to take two photos of the same scene a couple inches apart to mimic the stereoscopic view of your two eyes. Tint the right image red and the left image cyan, as below (in Photoshop, you can convert the images to Grayscale and then Monotone). Overlap them transparently (in Photoshop, "Multiply" the top layer) and you're done. Quick and easy. In theory.
Two pics from slightly different angles stand in for your left and right eyes.
In reality, it can be tricky to get the two layers to line up right. The depth of field--which parts pop out and which recede--is determined by how you match up the images. Any little difference between the photos, like if you tilt the camera a bit or the wind blows a branch, can ruin the illusion. I've learned that choosing the right subject and being the right distance from it is important. These three experiments turned out best, but I've got six or seven others that hardly worked at all. I also tried some techniques that give a more full-color effect, but I wasn't happy with those either.
Whenever I'm in an antique store, I love checking out those old stereo viewers and cards that were all the rage in Victorian times. The original Viewmasters. I could imagine myself going nuts collecting hundreds of those cards someday. The red-blue technique's not the same, but seeing a picture pop off the page is just as magical as ever.