Weatherstrip

Open the door and pull the old strip back at the bottom of the lock side. Most strips push into a thin slot (a kerf) in the frame.

Hold a fridge magnet on the edge of the door where the handle is. Steel doors, and many wood and fiberglass doors with a metal edge, will hold it.

Not needed: strips for your frame aren't made with magnets.

Close the door. From inside, measure from the face of the door stop to the face of the door (see how). This is the space the strip has to fill.

No tape handy? Use the daylight test. No daylight, just a worn strip: standard reach. Daylight around the door: high reach. Still daylight after high reach: MEGA reach.

Strips trim with scissors, so pick the length that covers your door.