This is a guide on how to catch Munchlax in Pokemon Sword and Shield. Although it is rare to see one, it is quite obvious where you can find them if you pay attention to the surroundings.
As soon as you cross the bridge to the right of Motostoke you will enter the Motostoke Riverbank. There is a Snorlax that spawns directly in front of the bridge, but on your first encounter it is too high level to catch. Instead start searching the nearby grass and keep an eye out for the Munchlax overworld spawn. It must be normal weather for Munchlax to appear, and it is quite short and can be quite difficult to see.
Keep running laps around the grass and one should eventually pop up. Whittle it down and capture it to add it to your team.