Purchase the correct replacement gasket for your your freezer make and model. Stretch it out and work out any kinks from shipping and storing in the replacement gasket by heating it gently with a hair dryer and smoothing out the gasket or by softening it in warm water.
Disconnect the electricity and empty the freezer, moving any frozen food into a cooler.
Examine the old seal to see how it is held onto the freezer. You may need to peel back the gasket to see below it. Some gaskets are glued on, others are held in place between the freezer door and liner. Clean the seal and the edges inside and outside the freezer door.
Remove the old seal. Peel off or cut away glued seals using a razor knife, cutting along top, sides and bottom edge. Loosen a few screws at a time to remove a seal held between the door and liner or under a metal retaining strip. Work on one side of the freezer at a time, feeding the new strip into the gap created by removing the old gasket.
Feed the flange on the new seal into the groove on the freezer, starting at the top of the freezer door. Press the seal firmly into the gap or groove, working across the top, down the sides and then across the bottom. For a flange that is held between the freezer door and liner, press the new seal into the gap created when removing the old seal, doing a small section at a time.
Pay special attention to corners, making sure the seal is properly aligned and seated in the corner.
Tighten any loose screws as you finish each side.
Test the door seal to make sure it is sealing when the door is closed. Make sure the door is lining up properly and is level. You may need to tighten the hinges if the door is beginning to sag.
Turn the electricity back on and restock the freezer.