The concrete mix will set quickly, so if you don’t want to be on the clock when coming up with the design for your stepping stone, it’s a good idea to map it out first. I just took the form and eyeballed the size, coming up with a design that I thought would work well.
I selected the flattest and smoothest stones I could find so the end product wouldn’t shred bare feet in the garden. I arranged the stones I liked the best in the middle of the design. Take as long as you’d like to come up with a design you’re satisfied with. But remember: If you mess up, you can just pluck the rock out, smooth the concrete over, and try it again.