I just installed 15 replacemenet windows and found it was much easier than anticipated. All that needs done is to remove the interior window stop. With a chisel, clear out the middle stop on the sides making a smooth finish for the repacemnt window. If the new window was measured and made correctly, it should slide in nicely. Just caulk around the edge put in place. Screw in the sides. Replace the window stop and your good to go.