Fishing (ALT-F) makes use of the appropriate lore & craft skill to catch fish (what else?). There are two kinds of fishing: passive and active.
Fishing is a good way to gather food, which makes it a viable choice for newcomers.
Active fishing[]
Active fishing is fishing using either a club, spear or fishing rod (in order of least effective to most effective). To do so, choose your fishing implement and pick a direction in which to fish. Fishing lasts for a long time, up to 3 in-game hours.
Active Fishing does seem to actively use your Fishing skill, and ritual Fisherman's request for a catch. Use the ritual right before doing the fishing. Try to trade for a good or masterwork fishing rod in the village because club or staff or spear/trident is not that helpful.
Passive fishing[]
Passive fishing is fishing using nets, which you must trade for, and can only be placed in deep water. After the net is placed (usually by boat or by swimming) you should check back on it in order to gather your catch. Checking is done through the fishing skill, and neglecting a net will mean its fish will rot. Use the fishing skill to set a net, and also to check it. After setting the net, wait for a minimum of one day and maximum of 2 days, then check it. Hopefully you will have caught something. It's best to arrange so that you check on every even or odd-numbered day so you don't forget. Multiple nets are a very good way of obtaining food and increase your fishing skill; however, if you forget to check it, the fish inside the net will rot, and you will have nothing to eat. Nets are only sold in certain villages' shops, therefore it takes time to find and gather them. You will start with two Nets if you take the "I want to be a fisherman" backstory, which can provide you with enough food to survive, meaning any extra you gather can go into storage in preparation for winter, or as an emergency supply if nothing is caught in a long period of time, or injury prevents you from gathering sufficient supplies.