Plastic, or vinyl, window frames give a home an updated look. They are easy to install and come in many sizes and styles. Before purchasing a new window frame, measure your existing window frame to see what size you will need. If you have a standard size window, you will be able to purchase a vinyl window frame directly from your local builders supply store. However, if you need your window custom made, you will need to order it. Many builders supply stores will take orders for custom windows.
Apply a bead of silicone caulk around the new vinyl window flange, on the side that will be facing the house.
Put the window into the opening from inside the room. Start at the bottom and angle it up into the rough opening. Make sure the window is in as far as it can go.
Hammer in one nail in the upper side jamb. Use a level to make sure the window is sitting straight. Push shims between the window frame and the rough opening, on all four sides of the window, to hold the window centre in the opening.
Nail through the window frame, shims and into the rough opening to hold the window in place. Trim off the extra shim with a utility knife.
Go outside and check the gaps around the window frame. If a gap is 1/4-inch or less, fill it with elastomeric caulk. If it is larger that 1/4-inch, you will first need to fill the gap with foam-rubber backer rod.
Check for any gaps on the inside of the window frame. Fill any you find with minimally expanding foam.