Jamie from Great Hilton High School sent in some pictures of his solutions. He says

When trying to make a square, there are 2 rules to follow, which are:
1. All the sides need to be the same size.
2. There must be 4 perpendicular angles (or right angles).

To complete the squares, see how many grid squares 1 line takes up and rotate the line 90 degrees. Keep doing this until you get a full square.