Step #1: Start with a loose sheet, draped evenly over the bed.
Step #2: Tuck the foot of the sheet under the base of the bed, from corner to corner.
Step #3: This is the step to practice. Grab the long edge of the sheet, and pull it up so that it wraps smoothly around the corner of the mattress. The sheet should now run straight down, perpendicular to the ground.
Step #4: At this point, the sheet should have two "wings" that extend above and below the mattress. Tuck the lower wing under the mattress.
Step #5: Next, fold down the upper wing, then tuck it under as well.
Step #6: Smooth out the edges and folds, and your military corner should be good to go.