Remove the Larson door from its hinges using the proper type of screwdriver depending on the screws. Place the door on a hard, flat surface.
Pry out the old spline from around the screen using the blade of a standard screwdriver. Once you have pried out several inches, you can generally pull out larger lengths of the spline with your hand. Remove the screen after you have removed all the spline.
Lay the new screen on the frame so that an equal amount of screen overlaps on each side. The replacement screen should be several inches larger on each side than the space in the Larson door frame. Ensure that the screen is laid flat without any wrinkles.
Insert the end of the spline into a corner of the frame, trapping the screen under the spline and inserting it into the groove in the frame. Use the spline tool to roll the spline on top of the screen and into the groove. Keep the spline stretched and the screen laying flat as you proceed. Insert the spline into the entire perimeter of the screen.
Cut off the excess screen from around the edges using a utility knife. Re-hang the Larson storm door onto the hinges.