If you are having trouble merging the two images, try this: place the edge of a large card against the screen between the pictures, pointing towards your nose, to help isolate the images properly. Start with your eyes about 1 1/2 - 2 feet from the screen. If you wear glasses for myopia, remove them -- it will help you get close to the screen and keep your eyes relaxed.
The trick is not to go cross-eyed, but rather the opposite -- to let your eyes relax as if they were looking at a distant object.
These are only a small sample of the hundreds of stereo photos I've taken in the last few years -- they're just the ones I happened to have in print form (slides are not easily scannable).