It's time for some assembly. On a large table (much like the one in our Free Woodworking Plans for a Portable Shop Table), lay out the rails and stiles in position face down on the table.
Place a thin bead of glue on the mating face of one stile where one rail meets the stile. Align the two pieces into their proper position and clamp them into place. Insert a screw through each of the pocket holes and into the stiles. Snug the screws so that the joint is tight and immediately clean up any glue that may have seeped out of the joint with a lightly-dampened cloth.
Repeat this step with the other three sets of pocket joints to complete the square of the mirror's frame. Check the unit for square by cross-measuring diagonally across the frame and comparing the measurement to the opposite diagonal. If the lengths match, your frame is square.