As epic said above, there is a specific product called "crackle medium". If your local paint store doesn't have it, try an art supply store. There are dozens of manufacturers who make it. I wouldn't recommend using the hide glue method, although it definitely works, it won't yield uniform cracks in the paint above.
Here's a link with a how to: Invalid Link Removed
I have found I can get better crackling in the top paint by painting the crackle medium on in a "swirling pattern" as opposed to straight brush strokes. The straight strokes will yield straight-looking crackles, whereas the swirl will make them much more random, like the guitar in the link you posted.