The base is 3" of compacted 5/8 minus crushed gravel. Then there is a 2" mortar bed into which the stones are poked. Once the area is filled with the stones, a piece of plywood is place over the bed (and there must be a screed of some type of material, leveled to help the board do its job of leveling the stones).

So must you use s flat surface to create this mosaic since you mention laying a board on top with weight? Do you plan or layout your design in some manner before you set the stones and how much time did you have from when the mortar was put down until it dried, how long to complete your work? Did you lay the mortar as you lay the stone? Thanks a lot.