I don't have a pic handy right now, but the fuselage should have a small hole where the "Big Hole' will go. The finished hole will ultimately be sort of halfmoon shape, having a semicircle on the side and bottom. Start small, and once large enough, put your pushrod on and operate the flaps to see where you need to do additional trimming. You need to have the rod operate without binding and the ball connector that goes into the flap needs to clear the skin. The hole will be in the area of about 3/4".
I used dremel cutters and sanding drums (1/4" and 1/2") to finish the holes. You get to hook up your flaps and test operate them. How exciting!!!
Have fun,
Roberta