Stir the joint compound with a handheld drill equipped with a mixing paddle until it is uniformly smooth. Pour the compound into a putty tray.
Scoop joint compound onto a 5-inch putty knife. Apply the compound to the joint in one stroke all along the joint. The joint should be covered with a smooth, thin layer of compound.
Cut a length of drywall tape the same size as the joint. Work the tape onto the joint, moving from one end to the other.
Cover the tape with a thin layer of compound and an 8-inch putty knife. Feather out the edges of the compound. Allow the putty to dry overnight.
Sand down the joint the next day with a sanding block or power sander until the joint is smooth.