Scrape off any old caulk insulation with a putty knife. Clean the scraped areas and the windowsill and sashes thoroughly with a rag and denatured alcohol.
Measure the moveable window’s top, bottom and sides. Cut four strips of v-strip weatherstripping to size, using a utility knife.
Open the window.
Peel off the adhesive backing of one of the v-strips. Press it onto the bottom edge of the moving window’s frame where it touches the windowsill when the window is closed. Peel off the adhesive backing of two more v-strips and press them against the sides of the window sash. Place the strips so that the open end of the “v” faces the interior of the house.
Close the window.
Peel off the adhesive backing of the last v-strip. Press it between the gap where the bottom sash of the top window meets the upper sash of the bottom window. Orient the strip so that open end of the “v” faces up.