How to Install New Window Screens?
Answers:
As Moonage said, if you are using wood screens, that will be flush with the trim, then you can use a simple fly, or butterfly clip.
If however you would like to use newer metal or vinyl screens, you have a couple of approaches... the simplest being to add a piece of trim at the top of the window... leaving enough space for the thickness of the screen (minimum), and another trim strip at the bottom. Most of the newer screens have a spring mounted at the top two corners... so you push the screen up into the upper gap, and over the bottom piece of trim. The spring then holds it in place. The top piece of trim should be at least 3/8" or larger, and the bottom piece needs to be no more than 1/8" high (or you won't be able to lift the screen over it.)
You didnt say what type of screens you are using, whether they are standard sizes or custom. (You can even buy the parts to make your own)
The advantage to the screens with a spring at the top, are that they are very easy to lift out and clean.... If you don't care about that, you can always shoot some screws thru the screen frame, into some trim strips around the window. (I would use 'hex head' or 'pan head' screws for this)
Have fun!
Other answers:
if the scrns are wood and flush to the exterior trim , use a fly clip which you screw to the trim and turn the metal fly to hold screens in place
if the scrns are wood and flush to the exterior trim , use a fly clip which you screw to the trim and turn the metal fly to hold screens in place