To install the gasket:
The gasket is shipped in flat cardboard box, with part in its own enclosure. Mark that part of the gasket before removing. That section has a magnet, should end up mounted at the top of the freezer door.
Lay out the new gasket on a flat surface. Then use the high heat setting on a hair dryer to soften up any deformed areas and use your fingers to get them right.
Pull out the old gasket out of the slot. You can work it off the freezer door without removing the door.
Push the new gasket into the slot. Make sure the magnetic part is at the top - that will help hold the freezer door closed.
Make sure the corners are square and completely pushed into the slot.
Related fixes:
6a. Use the hair dryer to remove all ice accumulated around the freezer compartment where the door closes. This helps ensure a good seal.
6b. Ensure the drawers work well. Use the hair dryer to melt all ice from the tracks, especially at the back of the freezer. That can keep the drawer from closing all the way. Then the door can't close completely and your freezer will ice up again.
6c. I also sprayed WD-40 on the drawer tracks. This made it much easier to open and close the freezer door.
Now our freezer is completely ice-free!