DIY & Home Improvement

First, you actually want to put the flagstone in the gravel, not the gravel between the flagstone. You'll need to dig it deeper than the thickness of the stone - at least 2" deeper, but 4" would be better. Put in most of the gravel, and then put the stone on the gravel. That way you can adjust the gravel so that the stone comes out even and there's no trip hazards.

Second, you missed compaction. You'll want to compact the gravel before you put in the stone so that the gravel doesn't settle. The easiest way to do that is to rent a plate compactor and run it over the gravel. It resembles a lawnmower, but with a solid metal plate on the bottom that vibrates. Your other alternatives are a hand tamp, or hosing it down and stomping the hell out of it and hoping for the best.

Third, if you want any plants to grow, you will need dirt instead of gravel. And keeping the weeds out of it could be tricky unless you plant a ground cover that will completely fill the spaces between the stones.