Take a sheet, cut or rip(go ahead ripping makes you feel better, lol) into 4-6 inch wide strips, you will fold these 3 times, so however wide you want your ties (X4), it really doesn't matter how wide they are, but I really wouldn't make them smaller than 4 inches.
iron strip in half to find the middle
unfold, and fold top and bottom to the middle crease
fold pillowcase and tie in half to find the middle
Match the middle creases and fold over tie and pin. Make sure you put the side that is open at the top in between the tie.
Here are all the ones I did! It took me 30 min to do 1. If your opening is in the middle like mine, all I did was do 1 straight stitch across and closed it up. I am also adding a cool gathering gardening thing where you stitch button holes on the sides with a button. Here is that tutorial....