When installing a flagstone patio in colder climates it is best to excavate 6 to 8 inches of the existing soil and adding a good base of aggregate and then compacting that very well fllowed by 1 to 2 inches of sand so as to set your flags on. the sand makes it much easier to set them. be sure you give this a slight pitch away from your foundation. In my experience in landscaping with flag, I dont think that sealing is critical unless you want a shiny look to it. I hope this is helpful.Mike