How can you treat a broken finger?

Depends on break: Very much depends on the details. At its most basic all fractures need to have the pieces in the right place (so-called reduction, if they are out of place) and then held in place (so-called fixation) reduction can mean anything from leaving the fracture as is to closed manipulation to open surgery. As to fixation this can be anything from a splint to cast to plating, pins and screws.

See a professional: If you think you broke your finger the smartest thing to do is get examined by a professional and probably have an x-ray taken. If it is broken and heals crooked it will have longlasting negative consequences.