The average house has as much lineal footage of inside corners as it does flat seams. A good system for taping and mudding inside corners improves the look of every room and saves time.
Applying mud (drywall joint compound) and tape to an inside corner is difficult because it often involves blending three corners, like where the inside corner of a wall meets the ceiling. For inside corners, I prefer to use paper tape instead of fiberglass-mesh tape because it is creased down the center, and is strong and easy to work with.
Before I tape inside corners, I tape and mud flat seams to ensure that the inside-corner tape laps over the flat-seam tape. Taping inside corners is done in three steps on three different days: one day for taping and two days for the finish coat.
After each coat dries, I use a pole sander with 150-grit paper to knock down bumps or rough spots. If you don’t have a pole sander, you could use sandpaper wrapped around a flat wooden block. After the final coat, I use 150-grit (or finer) paper and sand more thoroughly.