Clean out the outside of the window screens where you will be hanging the solar shades. Use a stiff bristled brush to remove any debris or leaves. Wash down the windows with a garden hose and sprayer. Wipe clean with a towel. This will help you obtain a more accurate measurement.
Measure the inside portion of the window, both from side to side and up and down. Record these measurements.
Measure the sides of the screen clips, if you're using external clips. These are the pieces that hold the corners of the solar frame together. Take the clips into account when determining the screen size, or the shade won't fit. Many clips are between 1/2 and 3/4 inch on either side.
Cut the sides of the screen to the right size, taking the clip sizes into account by subtracting them from the window side measurement. For example, if the window frame is 24 inches wide and you're using clips 3/4 inch in width on either side, the frame side should be cut to 22 1/2 inches (3/4 inches x 2 is 1 1/2 inches. 24 inches minus 1 1/2 inches is 22 1/2 inches). This should be done for the width and height of the screen. Mark the proper spot on the shade brace and saw it off with a hacksaw. Sand down any burrs with sandpaper or a file.
Fit the edges of the solar shade with the window clips. Larger screens may come with an extra brace that is clipped across the center. This brace should also be cut to size. The clips used for these braces, however, are often flush and don't need to be compensated for when cutting.
Place the screen on a flat surface. The floor works well for this. The narrow channel on the back of the screen should be facing upward.
Unroll the solar screen out on the frame. Cut it loosely to size with scissors.
Lay out the spline along the length of the channel with the screen beneath. Press the spline into the channel with a spline roller. Slowly roll the spline roller around the frame, pressing it into the channel as you work around the frame until the spline is pressed into place around the sides. .
Trim off any extra spline with a utility knife. Also trim off any extra bits of screen sticking over the edge of the spline.