Fold the edges on each side of the bottom left corner to make a hem. Press it in place with an iron. Make the hem as wide as you like -- usually 1/4 to 1/2 inch. Open the hem so you can see the crease.
Turn one corner in so the point is facing into the fabric, creating a right triangle. Press it with an iron. Reopen the corner.
Fold the corner you're working on in half by pulling the bottom right corner up and across to meet the fabric on the long left side of the runner. Smooth the fabric out to create a new triangle. Press the fabric with an iron. Sew a short line from the point where all the crease marks meet, down to the bottom, short end of the fabric.
Cut on the outside edge of the sewn line 1/4 inch from the seam so the cut is parallel to the seam. This removes the pointed corner.
Unfold the fabric. Press the fabric smooth.
Repeat on the remaining three corners.
Finish the table runner by folding over the hem along each side of the fabric, pressing it in place and sewing a seam.