For the final steps, secure the top supports to the upper center of each apron, flush with the top, using glue and screws (pre-drill the holes!).
Then, lay the top on a flat, smooth surface and position the frame, upside down, on top. The top will overhang the frame by 1" on all sides. Once you're happy with the placement, make pencil marks on the underside of the top to mark the position of the frame. Flip the frame back over and apply glue to the top of each of the supports and the tops of the legs. Turn the frame upside down again, and align it with the pencil marks on the top. Secure in place with screws through the supports into the top.