4.

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.

5.

The squares which are positioned near the edge need to be trimmed slightly (as shown in the photo) before sticking them on (if they are not trimmed they might keep on detaching). I suggest trimming the tiny pieces (a few mm ones) to their final size (so that you don't need to trim them later; the tiny ones are difficult to trim later).

6.

Press the surface of the squares from time to time with the smoother. Don't move the smoother around the surface, because that might move the squares. Just press gently from the top (press, release the pressure, change the position of the smoother, press, release and so on).