Start with a piece of yarn threaded on your needle and knotted.
Come up through your fabric. Pull the yarn all the way through.
Pretend you have an invisible square on your paper. Go down in the OPPOSITE corner to make a diagonal line.
Come up through the fabric in the left corner.
Pull your yarn all the way through.
Go down in the opposite diagonal corner. Pull your yarn all the way through and you should have an X.
Come up next to where you went down.
Make another diagonal stitch.
Come up in the left corner of your imaginary square.