Open the box of joint compound and rip open the plastic bag, then dump the joint compound into the 5-gallon bucket. Leave enough room at the top of the bucket so you can add water without it overflowing.
Insert the paddle mixer into the end of the drill and tighten it down, then place the paddle mixer in the bucket.
Fill a pitcher with water and have someone else pour some of the water into the bucket.
Start mixing the mud with the drill and paddle mixer, rotating it around the bucket to mix the mud and water. While you're mixing, have the other person gradually add water to the bucket until it's the proper consistency. It should be runny but thick, similar to latex paint.