Test fit the retrofit window in the window opening. If it is the correct size, insert a tube of door and window sealant into a caulk gun. Apply a continuous bead along the window opening's side and top edges, on the exterior side.
Lift the retrofit window into the window opening. Hold a level against the retrofit window's side and top edge. Slide wooden shims between the window and window opening to hold the retrofit window plumb. Place the shims at least 4 inches from each corner and spaced 8 to 12 inches apart.
Drill 3/8-inch pilot holes through the retrofit window's vinyl sides. Place the pilot holes 4 inches from the top and bottom corners and 8 to 12 inches apart, running along the sides. Do not drill pilot holes in the retrofit window's top or bottom edges.
Drive a galvanized flat-head screws through each pilot hole to secure the window in place. Score the wooden shims with a utility knife where the extend past the window. Wiggle them back and forth until they break off.