The first step is to prepare the rough opening. It is important to make sure that you have the right measurements. The framed opening should complement to the size of the window. The opening needs to be about one inch bigger than the window from all four dimensions: top to bottom and side to side. To be sure that the size is right, test fit the unit to make sure it fits.
After your frame has been prepared, it's time to place the new casement window in the frame. The window should fit snug in the frame. With the help of your level, be certain that your new casement window is placed accurately in the center, leaving the same amount of space on all four sides.
If the window is still not level, addition of wood shims will help restore the level. By commencing with the right side of the window, plumb with the level. Next, put another screw into the bottom right of the window. If the window opens without any blockage, the left side can be attached. If it does not swing freely, check that the window is square and plumb again. Recheck to insure if the window works properly by opening and closing it.