This is not something you want to start to do. They should express their own anal glands when they potty. A symptom of their being full is usually dragging their behinds on the floor.
Don't rush out to get their anal glands expressed though, because they should be able to empty them on their own. Once a vet starts this they usually won't be able to empty them on their own. Just leave well enough alone is my motto. Also, someone who doesn't know what they're doing can damage them.