
The two should be able to lock tightly together.

Trim the edges of the horizontal molding to assume the shape of the vertical molding, then snap it in place. Mark where this molding meets the ends of the vertical pieces. Then, measure and cut another piece of the molding and place it over the top casing. Start by measuring and cutting your vinyl molding to the right size and installing the vertical molding on both sides of the window. Make sure to trim the top casing to match the top edge of the vertical casing so the two can lock in perfectly.
#REPLACE J CHANNEL TRIM BUILT IN WINDOW INSTALL#
Once you are done with the vertical sides, install the casing on the top of the window using the same procedure. Now, install the casing and repeat the process to cut and fit casing to the opposite side of the window. Place the vinyl casing on one side of the window and trim the bottom so it follows the angle of the sill. Once it is secure, attach the end caps to the edges and glue them in place. Place your new vinyl windowsill over the old one and nail it in, positioning your nails about 6 inches apart. Use a pair of tin snips to trim it off and leave the current windowsill in place. Remove the metal flashing installed over the windowsill too. Do this carefully especially when removing the siding, as you will need to put this back after you have done the installation. Pull off the existing J-channel and siding panels from around the window.
#REPLACE J CHANNEL TRIM BUILT IN WINDOW ZIP#

You can get the job done with simple tools and materials like: The good thing about this project is that you don’t need to hire complex equipment. Materials Needed for Installing Window Casing for Vinyl Siding
