Check if the squares are firm enough (you need to be able to pick them up and stick them on so that they don't lose the shape. If they are firm - apply some glue (thin coat) on a small section of the board (f.ex. 1/5).
Start sticking the squares on. Make sure that they are positioned straight and in one line. Use the metal ruler to line the squares up if there's need (don't press too hard - you don't want to damage their shape). Don't press them to the board. You will press them from the top with a smoother later.