The folks at eHow will help you out with the instructions. These steps are for the traditional blanket and bedspread.
1) Put the bottom sheet on the mattress. If it is a fitted sheet, it will have sewn-in corners and you can fit it snugly over the mattress. If it’s a flat sheet, follow step 2 to make hospital corners.
2) Lay the top sheet on the mattress so there’s enough to tuck under the mattress on all four sides. Tuck the sheet snugly under the mattress at the foot of the bed and the head of the bed. Tuck the corners under the mattress on the long sides, making sure that the folds are flat. Tuck in the sides, pulling the sheet taut.
3) Place the top sheet over the fitted or bottom sheet, and again make hospital corners, but this time only at the foot of the bed.