Remove the screen frame from the window
Inspect the screen for tears, holes, bent frame, or loose spline
Clean the frame and screen groove
Cut a new piece of screen mesh slightly larger than the frame
Lay the mesh over the frame and secure it with screen spline
Use a spline roller to press the spline into the groove
Trim the excess mesh with a utility knife
Straighten or replace bent frame corners if needed
Replace damaged corner pieces or clips if broken
Reinstall the screen into the window frame
Test the fit and make sure it is secure