Good luck!
Measure the inside of the window trim. (On the outside of the house). Take those measurements to the nearest Ace Hardware store. Tell the clerk you want to make window screens to fit your window. He will sell the materials to you.
You will need, a sharp knife (razor knife works best), hacksaw, tri-square to make 45 degree markings, screw driver to screw in the clips to hold the screen in place.
Take the aluminum frame material and mark a 45 degree line on one end with a pencil. Make sure when you have cut the end off, the long end is outside, not the side with the channel the cord and screen fit into.
Using the hack saw, cut the end off at 45 degrees. Measure the frame material the length you need for the side of the window. Make another 45 degree cut. Make another piece just like the first.
Using the same process as above, cut two shorter pieces for the top and bottom of the window. Lay the pieces on a flat surface and stick the corner braces inside the frame material. You should now have a complete square aluminum frame.
Lay the screen material over the frame. Take the cord and push the cord into the groove in the frame. The cord holds the screen into the groove. Push the cord in all the way around. There is a tool made just for this purpose that cost about $3. Buy it when you buy your materials. It is worth having for repairs, etc.
When you have gone clear around the frame pushing the cord and screen into the groove made for it, take the sharp knife and trim off excess cord, and then just run the tip of the knife around the window to trim the screen to fit.
Instal the screen in the window opening and screw the littl tabs that hold the screen in place into the window trim. You are done. Time used about 15 - 20 min. per screen.