After the bottom row was set, I let it harden for a couple of days. Then I started to stack the stones in small sections at a time to see how they would fit together. I usually did this before going to work every day, so I would do about a third of a level each morning.
Once I liked the way it looked, I moved the stones off to the side, being sure to keep everything in the same sequence that I had just created. Admittedly, this was the hardest part for me and took a long time. My best advice is to not get caught up in perfection.