Establish the size of your mattress and bed. If you don't know this, use a tape measure to gather the dimensions of your mattress, and consult a bed size chart. Beds in the United States range in size from a narrow bunk bed to a grand king.
Visit your local bed linen store or go online to purchase a flat sheet to fit your bed.
Remove the fitted sheet from your bed and place it in your laundry.
Take your new flat sheet out of its packaging, unfold it, and iron on a medium heat.
Lay your flat sheet over your bed.
Lift one side of your mattress, and tuck one side of your sheet securely in place. When you reach a corner, tuck the sheet in allowing it to overhang the next stretch of your mattress.
Tuck the overhang under your mattress and then repeat the process around the perimeter of your bed.