Answer: You can use mortar or you can pack sand between the flagstones. Mortar will make the steps more stable in the long run, but if you pack sand or a sand/soil mix between the flagstones and then plant a stepable ground cover such as Irish Moss or Wooly thyme, the roots of the plants will knit the sand/soil together and make for a sturdy step. Either way, I'm sure your project will be fantastic. Enjoy!
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