Hair mousse is a finisher that can be used in different ways to achieve different effects, as well as being made for various hair types.
Although this can vary a lot from one formula to another, many can be used both on still damp and already dry hair. Either way, the recommendation is to apply it on the entire length of the hair, from root to tip.
Using a good wooden comb is also a great way to help you distribute it well through your hair, already shaping it according to the hairstyle or effect you wish to create.
In case of doubts, remember to always check the packaging, the manufacturer's recommendations and how to use it.