For this project the only things you need are a blanket and some string/rope.
Step 1: Pull Off the Old Sheet.
Lay out your blanket in the floor, make sure there is enough room to spread it out completely.
Step 2: Knot the Corners
Step 3: Tie the Corners Together.
Pull this taut and tie it to the next corner.
Continue on all four corners of the blanket making sure they are as tight as they can be.
Step 4: Put It Back on Your Bed
This is the easiest way to do this that I have come up with so far... Of course you could take more precautions with your blanket and add some plastic rings to separate the blanket and the rope so you don't run the risk of tearing it. It's all up to you.