Wood Window Installation Tips
The trick to installing the replacement windows lies in the initial measurements. Take your time and make sure you measure correctly. Always check the width at the top, bottom and middle. Use the smallest measurement. You can always shim out the jamb liners where the opening is "fat" (a carpenter word).
Check the opening for square using an accurate carpenter's square. You can also attempt to measure the diagonals of the opening. The diagonal is the straight line from one corner to another. For example, when looking at a window opening you create a diagonal if you stretch a tape measure from the upper right corner down to the lower left corner of the opening. If the diagonals are equal, the opening is perfectly square (this assumes that the width and length dimensions match as well).
When it's time to install the window, take your time removing the window-stop molding. This is the molding on the inside of the window opening that produces one half of the groove that the inner sash slides up and down in. The odds are this is an old house with multiple layers of led paint. You need to be careful about cleaning up any paint chips. Use wet paper towels, not a wet-dry vacuum.
Use a small pry bar to carefully pry this molding away from the window frame. If this is painted, consider slicing a fine line with a razor knife to break the paint film at the intersection point between the molding and the frame. If the stop molding was nailed in place, don't hammer the nails back through the molding! If you do, you will have massive blowouts of wood. The better way is to use a linesman's pliers and pull the finish nails through the back of the molding. It can be done with little effort.
Give serious consideration to prefinishing the new windows. I did this on virtually all of my jobs. That way, no ladders are required to paint the exterior. It also minimizes the chance of a major paint spill inside the house. Besides, it is so much easier to paint the windows in a garage on a sawhorse. Don't throw away the old windows! An architectural-salvage company in your area may buy them. People use the old glass and window sashes for historical preservation!