First, use a putty knife to remove any loose material near the damaged areas.
Pour some dry joint compound powder into a large bucket and begin adding small amounts of cold water while thoroughly mixing the compound until it reaches the consistency of cake batter. A drill with a mixing paddle attachment will greatly speed up mixing time. This process of allowing the mix to cure and change from a powder to a useable putty is called "slaking." Cold water should be always be used as it helps slow down the curing and hardening of the compound.
Allow to mixture to sit for one minute before proceeding.