Once you've got your window level, you need to make sure it's also flush to the inside wall. To do this, line up your set square against the wall and either push the window out or get a person outside to push it in, until the wall and window are level.
Pre-drill three holes into each side of the reveal at the top, middle and bottom. Use your set square to make sure the frame stays flush with the wall. Then drill in three screws to attach one side of the reveal to the wall. Now you have one side of the window attached, you won't need help to hold the window in place.