Measure the existing window opening
Order the correct replacement window size
Gather tools and materials
Remove the interior stop or trim
Remove the old window sash or unit
Clean the opening thoroughly
Inspect and repair any damaged framing
Check that the opening is square and level
Apply flashing tape or waterproofing as needed
Dry-fit the new window in the opening
Insert the replacement window
Center and level the window
Shim the window until plumb, level, and square
Fasten the window according to manufacturer instructions
Recheck alignment before fully tightening fasteners
Insulate gaps around the frame with low-expansion foam or backer rod
Reinstall interior trim or stops
Seal exterior edges with exterior-grade caulk
Install or replace exterior trim if needed
Test the window for smooth operation
Clean the glass and surrounding area