Attach a stop block with a 45-degree mitered/angled end to the guide to hold the poplar at the correct position while mitering the frame components. For a rectangular (versus square) frame, you will have two different lengths for sides versus top/bottom and, thus, two different stop block positions. Take the measurements using the outside dimension of your photograph/mat, subtract twice the width of the rabbet, and then cut the miter from the inside of the frame piece outward.