After adding the rocks, sprinkle the sand with larger grains.
Cover all the area you like to have large sand grains.
Then apply the sand with smaller grains.
Cover the entire surface with small sand grains.
Start again on another zone of the board.
If you work on a large board, it's easier to divide the board in small working zone so that the glue will not dry before you cover it with sand.
Here I clean the textured plastic tiles so that the sand will not stick on it.
Again, start with the rocks and larger grains sand.