Welcome to the Workshop community @Pooja1. Many thanks for your question, I'd be happy to assist.
Our wonderfully talented community member @Tara86, has recently added a step by step D.I.Y article on How to replace a flyscreen. I'm sure you will find it helpful in your project.
On the topic of making flyscreen frames, here's how to go about it.
Take your existing flyscreen and measure both the height and width. If they are too damaged, you can take measurements directly from the window. Make sure the window is square before taken measurements. You need to make an allowance of up to 3mm so the frame fits the window. If there is a top track, you also need to make an allowance for the screen slotting up into it and dropping back down into the bottom track.
Measure the frame piece along the longest side tip to tip, as this will be the outside of the frame. Make sure the frame is channel side up in the mitre box and the channel in on the inside of the 45° cut. Use the mitre box to cut a 45° angle. You can screw the mitre box down to some timber to aid accuracy. Proceed to cut the other 3 sides of the frame.
Take your four sides and insert the frame corners into them. They should be a snug fit. Ensure the 45° cuts line up and meet each other. Test fit the frame in the window, to see if any slight size adjustments are needed.