What is reticulation?
Reticulation, in photography, is the process of fracturing the film for creative effect. The effect itself is akin to the fractures in shatterproof glass: cracks that form after it shatters but before it actually falls apart. The word reticulation outside of photography means, as Webster so deftly put it, netlike. If you can imagine the broken shatterproof glass again, it does look vaguely netlike and interconnected.
How is reticulation created?
Reticulation is created by heating film negatives in water to a high temperature, then plunging them into an ice bath to fracture the emulsion by way of differential expansion. I've also heard the word reticulation used to describe less dramatic warping of the negative by the same technique, not just the cracking. In any case, the ideal photographic subjects for reticulation are bold and not too dependant on detail, since tearing apart the emulsion isn't kind to delicate, detailed images. The effect is akin to what happens when a piece of glass or ceramic is heated and suddenly plunged into icy water: the sudden forces on a relatively brittle medium tear it apart into a fine lattice of hairline cracks. Depending on a few variables (the temperature of the hot and cold baths, the specific film emulsion, how much time the negatives spent in water beforehand), the effects can vary.
Caveats and Advice:
I've only ever heard of reticulation on photographic negatives, not final prints. The process may not be possible on printing paper, or it could just have an unattractive effect - I've never tried it. This process necessitates "ruining" a perfectly good roll of film for the sake of the effect, but it's just as easy to shoot a roll of film specifically for the purposes of performing reticulation. Note that I'm using the word negatives here only for convenience: there doesn't seem to be any reason why this wouldn't work equally well with slide film.
Also note that I have never done this myself. My knowledge of the process comes from being a photography student and hearing about it from teachers.
I can't think of a reason why reticulation couldn't be created after the film negatives are processed, but all the instructions I've seen for it have listed the reticulation as part of the development cycle. The most standard instructions seem to use the washing process, since high temperatures at development yield large grain and unpredictable results. Cold fixer might not be nearly as effective as room-temperature fixer (I honestly have no idea), and at least one washing cycle at normal temperature seems necessary to avoid long-term degredation of the negative by any fixer left behind.
Interestingly, reticulating photographic negatives has gotten much harder over the years as film manufacturers have made negatives more and more durable. As I understand it, it's now necessary to almost boil the water and film before icing them down in order to produce the desired 'fractured' effect. Then agian, heating negatives excessively could cause them to dissolve in your hands. Get ready to experiment, because your mileage will certainly vary.