I placed one hole centered 10" from each end, and 1.75" in from the edge.
The other hole in each pair of holes was 3.5" from the first hole. I
measured and marked the centers.
Then I clamped a 2x4 underneath the plywood so that the forstner bit would have something to drill into when it finished going through the plywood. This prevents "tearout" on the underside of the plywood. Without using a piece of wood under the plywood, the drilling process will destroy the wood on the underside -- you'd be very unhappy with the results, and it may leave hazardous splintered wood.