Screw flyscreen mitre box to a bench or saw stool to hold it steady.
Put frame, channel-side up, in mitre box. Use hacksaw to cut a 45° angle at 1 end of frame so channel will be on inside of mitre.
Measure length of frame piece from long point to long point (this will be the outside of a frame side) and cut a 45° angle in opposite direction. File cut metal to remove metal burrs. Cut other 3 sides of frame.
Insert corners and push frame tightly together. Check fit of frame in opening.
Put frame, with spline channels facing up, on a flat surface. Roll out mesh over top and align weave with frame. If it’s too large, cut so there is about 50mm overhanging all around. Use spline and spline roller to force mesh into grooves to hold mesh tight. This action will also tension mesh. If it goes wrong or wrinkles, simply pull out spline and do process again. If spline is too hard to press in, try placing it in hot water first to make it more pliable.
Use utility knife to trim excess mesh on outside of spline. Add catches and clips to screen or window as necessary and fit screen in place. Repeat process for other screens.