Decide on the type of sheetrock mud that you want to use. The product types are based on drying time, such as 20 minutes or 90 minutes.
Place a strip of drywall tape on the drywall over the seam. The tape can be cut with a utility knife or ripped by hand. Press on the tape to adhere it to the drywall.
Add a small amount of water to the sheetrock mud that is in the box or a bucket and mix. The mud should be free of lumps and have the consistency of cake icing.
Apply a small amount of the sheetrock mud over the seam with the drywall knife. Skim over the mud to smooth it out and then let it dry.
Sand down the edges of the mud around the tape with the sandpaper to remove any high spots.
Apply a second coat of the drywall mud over the tape.
Allow the second coat of mud to dry. Sand the edges with the sandpaper.
Apply a third coat of sheetrock mud, and lightly sand with the sandpaper when dry.
Wipe the walls with a cloth or rag to remove any dust.