When you see pigs on TV and in old movies, they're usually eating out of a long trough filled with slop and kitchen scraps that's placed in the trough every morning by a farm hand. Instead of manually feeding the pigs every day, most modern farms use feeders that feed the pigs as needed. These devices consist of a large bin that holds the feed that is connected to a trough. Commercially produced feeders are made of metal and can be expensive, but they can be made out of plywood for much less money.
Design the feeder. Look at commercially built feeders beforehand to get an idea of how they are built. Typically, they consist of a trough connected to a large bin that holds the feed, which flows into the trough by gravity. The place where the bin and trough are connected is a narrow bottleneck that restricts the flow of feed, so that more food falls into the trough only as it is eaten.
Design lids for the feeder. These will help keep moisture and vermin out of the feed. The lid for the bin is a rectangular piece of plywood exactly the same size as the opening of the feeder that's screwed to a larger board with a handle attached. The cover for the trough is just a panel of plywood attached to the feeder by a hinge. The edge of the panel should overhang the trough slightly so that the pigs can lift it up with their snouts.
Draw out the pieces of the feeder on the plywood using your measuring tools and cut them out.
Assemble the parts of the feeder. The panels that make up the sides of the feeder should be screwed into the edges of the panels that make up the front and back. Drill a few pairs of holes in the back for tie wires so the feeder can be tied to the fence.
After installing the feeder, you should prop the lid to the trough open until the pigs learn where the food is. After that they will lift the lid up themselves to get to the feed. Periodically the trough will become clogged with a cake-like mixture of feed dust and pig saliva and will need to be scraped out so the feed will flow freely.