I was recently faced with installing windows in a new house, all by myself. Here's how I did it, First I cut a couple of 2x4s about 8 in. longer than my widest window. Then I measured the distance that the windows projected from the house and cut four blocks equal to this distance plus 1/4 in. I screwed one block to the ends of each 2x4.
Working from the outside, I installed one of the vinyl windows and placed the 2x4s over it. I screwed the 2x4s to the wall on both sides of the window, as shown in the drawing. Now I was free to go inside and level the bottom of the window and center it in its opening with shims. The extra 1/4 in. allowed me to move the window without interference from the 2x4 crossbars. When the window was centered and level, I went back outside and nailed it in place, beginning with the bottom flange. Then I pulled out the crossbars and moved on to the next window.
Scott Bruce, Grand Blanc, MI
From Fine Homebuilding 85, pp. 32
January 1, 1994