Set the repetitive stop on the miter-saw stand and cut all components of the same length at the same time. Label the pieces as they come off the saw.
After coating the cut edges with primer, run a bead of sealant along the long edge of the ripped piece.
Place screws at a 15-degree angle using a Kreg jig and a specialized stepped drill bit with a stop collar.
Place the pieces for each frame face down on an assembly table and put a bead of sealant in the joints.
To make outside corners the same dimension on both sides, set the blade at a slight angle (2 or 3 degrees) to create a tighter-fitting joint.