Cassio -

They're pretty easy to make. The best time to do it is before you join the wings together. What I did was mark a line parallel to the trailing edge, with the flap chord the same as the aileron chord (width). There is a rear spar in the wing which aids in the flap making process. Simply cut a little behind the line... take some scrap balsa and fill in the TE of the wing and LE of the flap.

Make sure you bevel the flap so it can extend downward... I made the bevel in this direction ---> \

Then I hinged it with CA hinges, but hinge points will work well, too. For the flap servo I made a hatch just like the ones for the ailerons. Cut the balsa away and add servo rails... then make a ply hatch for the servo with the arm sticking thru the side, just like the aileron.

hope this helps,
Scott