Cut away the sagging tape. Cut the tape one foot to either side of the sagging area. Remove and throw away the tape.
Sand the area down with the moderate grain sandpaper so that all ridges are removed. You want to sand almost to the drywall surface so there is a recessed space where the new tape will go. Vacuum away any dust.
Cut the new drywall tape. Cut it just shorter than the tape section you have cut away. Apply a thin layer of drywall joint compound to the area you have prepared. The layer should be uniform, free of air pockets or gaps and it should extend about two inches to each side of the area you have prepared. Lay the cut drywall tape on top of the joint compound and smooth it into place with the taping knife. Take care that there are no wrinkles or air bubbles under the tape.
Apply a second thin layer of joint compound over the tape with the taping knife. Hold the knife so the blade is flat against the surface and be sure that you do not move the joint tape. Allow the tape and joint compound to dry overnight.
Scrape away any high areas of dried compound with the knife. Sand down the compound to a smooth surface. Apply another thin layer of joint compound and smooth it over the entire repair area. Allow it to dry thoroughly. Repeat this step, using finer grain sandpaper each time until the repaired area is level with the wall.
Sand the final layer of joint compound with the fine grain sand paper so that the wall area is smooth and the repaired area is indistinguishable in level from the rest of the wall.
Vacuum the dust from the wall. Wipe it with a damp cloth to remove any remaining dust particles. Paint the wall to cover the repair.