Here is how to sew a square pillow:
Cut out your fabric. I used a quilting ruler and some leftover fabric. I cut two squares the largest size that I could.
Put the pieces right sides (outsides) together and sew around three sides, leaving one side not sewn. You can sew 1/2 inch or 5/8 inch from the edge.
Turn the pillow right side out and press flat.
To be more advanced, before turning right side out, cut the points off the corners, and fold over the open edges so you can iron them flat. Then, then turn right side out.
Stuff the pillow! (Edges are not folded in and pressed in the above picture.)
Here, you can see that the edges have been folded inwards and pressed.
Pin the open sides together and sew as close to the edge as you can.