With a miter box: A power miter saw provides great accuracy, but you can make miter cuts with a back saw and miter box. With either, mark the 45-degree cut line, then hold or clamp the piece while sawing.
Correcting angle: After sawing both pieces to 45 degrees, fit them together to check the angle. If the cut was slightly off, correct it by placing a tiny shim against the fence or inside the miter box, then saw again.
Clamping: A miter clamp holds the two joined pieces together while the glue dries. It also keeps the pieces joined if you want to strengthen the joint with brads or finishing nails after the glue dries.