Sian, short-sheeting is done with the top sheet. It requires the bed be made up with a blanket or bedspread to cover up what is (and isn't) underneath. With the bottom sheet installed normally, lay the top sheet on the bed with its "foot" end (with the small hem) at the top ("pillow") end of the bed. Tuck it in at the top and sides as if it were the bottom sheet, going about halfway down to the foot end. Now fold it back up into the position the top sheet would normally have, with the pillow end (large hem) in its regular top sheet postion, then install the blanket normally (I think most people fold the large hem of the top sheet over the top end of the blanket, somewhere around the middle or foot end of the pillow). It should look just like a properly made-up bed. The victim will usually turn back a corner of the top sheet/blanket combination and stick his feet in, expecting them to reach all the way to the foot end of the bed.