Humble plastic wrap can be used with watercolor in a variety of ways: to suggest the feeling or look of a natural element, such as ice or rocks; to create an interesting, non-representational area in a painting; or as the starting point of a new painting.
For any of these uses, begin by putting down a wet-into-wet wash, either in one specific area or over your entire surface. Then lay plastic wrap or heavier plastic sheeting over the wash. You can lay it flat, crumple it up or drape it in folds or other patterns. Lifting the plastic off fairly quickly will result in a softer texture, while leaving it on for longer periods will make more of a definite imprint. You can even weight the plastic down for greater emphasis and unique effects.
Each pigment reacts differently to this process, yielding a variety of textures. The results will also vary depending on the weight of plastic used and the painting surface you’re working with. The only way to find out what results you’ll get is to experiment, experiment, experiment.