Determine the top of the window unit, noting the sash operation. The expander should be installed on the bottom of the unit. (Note: some models have the expander installed at the top of the unit for shipping only. Remove the expander from the top and reinstall it at the bottom in this case.)
Position window into opening to check for a proper fit.
(optional) Remove the storm window and caulk the opening or surface on which the storm window will be mounted. CAULK THE HEAD AND JAMBS ONLY. Note: To allow for proper drainage, do NOT caulk the sill.
Reposition the window into the opening with the top of the window pushed up snug with the top of the opening. Temporarily secure the top corners of the window with two (2) installation screws provided.
Adjust expander so that it makes even contact across bottom of existing window sill.
Square up the window unit, then install the remaining installation screws making sure the operating clearance or “gap” between the window sashes and the window frame are even (the ideal “gap” is 1/16” on each side of the window sash).