A chest freezer is heavy, wide, and holds a surprising amount of ice and water. Defrosting it completely is the single most important step, since a full-size chest freezer can hold gallons of frozen buildup that will leak everywhere if you skip it.
Pro tip: Start defrosting two days out for a large or older freezer, since thick ice takes longer than you expect to melt.
Watch out: Never use a metal tool to chip ice. The refrigerant lines run through the walls and a puncture ruins the freezer.