Inspect the window frame for extending pulleys. If your frame has them, either remove them or use a hammer to knock them into the frame so they do not interfere with the installation of the JELD-WEN vinyl window.
Apply a bead of sealant around the frame's outside stop, the overhanging piece around the frame's exterior edge. Lift the new JELD-WEN vinyl window into the opening and press it against the outside stop.
Insert wood shims between the window's sides and the frame at each pre-drilled screw hole. If your window does not have pre-drilled holes, place wood shims 6 inches from the window's top and bottom corners, centered next to the window and between those shims spaced approximately 8 inches apart. Use a 1/8-inch drill bit to drill pilot holes through the window and shims.
Squirt a dab of sealant on a 2 1/2-inch corrosive-resistant pan head screw and drive it through the pre-drilled holes. Repeat with each remaining screw hole. Sink the screw's heads slightly beneath the vinyl window's surface. Push a screw cover over each screw head, to hide it.
Score the wood shims along the point where they extend past the window frame, using a utility knife. Bend the shims back and forth until they break off at the scored line.
Stuff foam backer rod into any gaps between the window and window frame. Cut the backer rod with a scissors, if necessary, to fit it in the appropriate spaces. Spread a bead of sealant over the foam backer rod. Go outside and apply a bead of sealant over the seam where the window rests on the outside stop.