Take some of the joint compound out of the bucket and set it off to the side. You can add it back in if you think you overdiluted the joint compound.
Add water to the joint compound a little at a time and mix. Keep adding water until the joint compound seems to be almost creamy. Add the water slowly but if you add too much and make the compound too thin add some of the reserved compound back into the bucket.
Pour the mixture into the hopper gun. This is a spray tool that has a big reservoir at the top. The material goes in the reservoir from where it feeds down into the spray gun portion.
Attach an air compressor hose to the spray gun. Follow all manufacturer instructions for the compressor and attaching the hose, as models and methods will vary from brand to brand.
Set up a spare piece of drywall, posterboard or paper and practice spraying. Use light pressure on the trigger and move the gun side to side fairly quickly to make a light layer. The ReliableRemodeler website says to treat the sprayer as if it were a garden hose.
Begin spraying sections of the drywall once you have the hang of using the spray gun. Apply two coats, waiting about 10 minutes for the first to dry.