A line is marked parallel to the flap's edge and intersecting the center point (B in the image.) Two perpendicular lines are then marked at both the points 2cm from the center point (A in the image.)

Cuts are made at A, then B. When cutting the latter, you will naturally want to avoid cutting off the tab (yellow dotted line.)

Note: Normally the lines shown in this image are drawn more lightly with the pencil, lightly enough that they can later be erased. The lines are drawn heavily in the images for the sake of illustration.