Everything will go much easier if you have your original wooden storm windows – just copy the dimensions. If not, determine the rail and stile dimensions based on the primary window, and build the screens to fit the opening minus 1/16″ on each edge. Then, if need be, scribe the edges prior to installation. Be sure to locate your mullion even with the meeting rails.
Start with 6/4 clear pine and plane to an inch and an eighth (or whatever the thickness of your exterior casings.) Note: you can’t get this wood at the big box; you’ll have to go to a real lumber yard. Use whatever joinery you prefer. I use double biscuits at the four corners and through dowels for the mullion. After glue-up, I use a router to create the inside profile.
Paint the frame before attaching the screen. Likewise, paint the screen moulding separately. This will create a much cleaner look and when it’s time to replace the screen it should be easier. Use a pin nailer to attach the moulding to the frame.
I like to use fiberglass screen. You can stretch it tight as a drum if you bow the wooden frame before stapling down the screen. Moreover, fiberglass won’t dent or rust. Staple the top and bottom. After you’ve released the pressure from the center, staple the sides, as well as the center.
A small detail, but make the mullion screen moulding thinner than the rail and stile screen moulding. This will eliminate gaps where the mullion moulding butts against the roundovers on the stile mouldings. Or to get fancier, route a cove on the ends of the mullion moulding so it mates seamlessly with the roundovers on the stile mouldings, kind of like a rule joint.