First, remove the entire screen assembly and lay it on the floor. Then remove the screen spline from the lower corner as shown.

Fit the screen adaptor frame into the lower corner of the screen frame. Note that the adaptor is sized to fit framing up to a maximum of 1/2" thick. 

Note:  If your screen frame is thicker than that, the work-around is to cut away the outside wall of the adaptor frame to turn the 'U' shape into an 'L' shape which will allow for screen framing of any thickness. Best tool for this would be a table saw set to a shallow cut.

Fasten the adaptor frame in place with sheet metal screws across the bottom and up the sides.