Hugh, I did this once and I guess that I was lucky or something since it did not seem to be a big deal. I had the cement company mix the the aggregate in which is really just what they always do. I used a big sponge and when it seemed to have cured enough, just washed off the top layer of cement with the sponge. You might want to set your forms up about an eighth of an inch higher to compensate for what you lose in the washing. It seems to me that using a hose too much will wash out the concrete around the stone and the sponge made it a lot easier to control. One thing that you do want to do is MIST the concrete a couple of times a day for 3 or 4 days so it doesn't set too fast.