Center the 12″ circle on the 14″ circle, then mark and drill the 12″ circle to create clearance holes for the studs and nuts. A 1″ hole saw is handy for this. Now test-fit the 12″ circle with a 28″ circle on top. Depending on your hub, you may need to cut out the center of this 28″ circle for clearance as well.
Glue and and screw the 28″ circles together with 1-1/4″ screws. Then screw the 12″ circle to the bottom circle.
Mark six 3/8″ holes on the 12″ circle, evenly spaced and about 1-1/2″ in from the edge, and drill them through the entire stacked turntable assembly.
Place the turntable assembly on top of the 14″ circle, then mark and drill matching 3/8″ holes through the 14″ circle.
Insert 3-1/2″ carriage bolts from the top down through all 4 plywood circles. Add washers and nuts beneath, and tighten.