The "return" on a handrail is a cap piece cut from the end of the handrail at 45 degrees. The cap is then glued and nailed back onto the end of the handrail. This covers the end grain of the handrail and wraps the profile or "returns" the handrail around the end. The return can be extended to the wall if desired.
Set the miter saw at 45 degrees to the right side.
Place the handrail on the left side of the miter saw fence. Cut off the end of the handrail.
Swing the saw back to 90 degrees. Flip the handrail upside down. Place it against the fence on the left side. Bring the blade down slowly, but do not turn the saw on. Get down at eye level and position the blade above the short point of the mitered edge, on the inside of the mitered edge.
Pull the trigger on the miter saw to activate the power. Cut down the mitered edge. When the saw has cut through the handrail, continue holding the saw down. Release the trigger while the saw is in the down position. When the blade stops spinning, let the saw return to its upright position. Remove the piece you cut off (the return) from the table and set it aside.
Swing the blade to the left-hand 45-degree preset; it will be opposite the first miter you cut. Flip the molding upright and miter the end.
Lay the molding on a worktable. Spread glue on the mitered end of the handrail. Stick the mitered end of the return against the mitered end of the handrail. Squeeze them together with your fingers until glue oozes out. Align the molding profiles. Wipe away excess glue with a cloth.
Predrill two holes through the return, using a drill and 1/6-inch drill bit. Hammer the 3-inch nails into the return. Set the nails just below the surface with a nail set. Fill the nail holes with putty.
If possible, cut two or three returns and choose the one that fits best.
The return is small and can get caught in the saw blade, so go slowly when cutting it. If it appears the return can get caught in the blade, release the trigger on the blade and let the saw blade stop without moving the blade up or down.
Always wear safety glasses when working with power tools.