The basic premise is that we all know that a small bore bit in a pillar drill will go through relatively straight successive diameters to the dowel size will be easier, through the whole 8 bulkheads will be relatively straight forward, then as each piece is plugged onto the dowels the larger hole is drilled, being a single bulkhead means less chance for the bit to snatch in a deep hole of various parts, once the first hole is cut through the 8 bulkheads, the set up is aligned with the next hole and the process then repeated for that hole.
I have a drill bit here that is designed similar to a wood bit, difference is that its diameter is adjustable, but unsure to what diameter, have never used it either as it cuts like a radar sweep, so a bit reluctant to use it, if I can find it I will photo it.