Begin by cutting all of your boards to length and drilling your pocket holes if you choose to use a Kreg Jig for construction. Set your jig for 1 1/2" material and place pocket holes in an unconspicuous location. For the Base pieces these are best at the inside edge of the shorter pieces and facing downward so they aren't seen. You will not need pocket holes on the legs but you will need them on the Frame pieces in a similar manner as with the Base pieces only you will also need them on the outside edges of the shorter pieces and the outside edges of the long piece. You will probably want to place these facing up so that they are hidden once you attach your table top.
To cut your 48" rounds you can begin by cutting your sheet of plywood in half so that you have 2 pieces that are 48" x 48". Then place a nail in the center (find this by drawing a line from corner to corner on the diagonal) and tie a string around it. Then tie a pencil to the other end so that it is just as long as the distance between your nail and the edge of your plywood (on a flat edge not a corner). Holding your pencil upright, pull your string taught and then draw a circle by simply running it around the outside edge in a circular motion, all the way around. Cut 2 of these out using a jig saw or band saw.
Once you have cut and drilled your pieces, you will lay them out and fasten as you see below in the diagram. You will use glue and 2 1/2" pocket hole screws to connect all of your pieces or you can use your countersink bit and 2 1/2" wood screws to fasten the legs to the Frame pieces. I wouldn't recommend using this method for attaching the shorter Frame pieces to the longer Frame piece if you can help it since you will have to come up and in at an angle and it's difficult to get your screw sunken deeply enough this way.
We are working from the top down in constructing the base frame of the table so you will flip your base frame over before attaching the table top to it.