i wouldnt use sand, for the reason it soaks up too much water and when wet its hard to work with. if there is sand already under your flags when you lift them,thats fine. go to a hire centre and get a vibrating plate or compactor plate. run it over the sand a couple of times,until its all tight. now get some quarry dust<fine crushed stone,like sand> about 4-5 tonnes. set screeds to the height of the bottom of your new flag. say 6 foot long and 6 foot apart.fill in between your screeds and level off with a board slightly longer than your screed width<6'6">. then plate the lot again and repeat the leveling process. now your new flags can be set into place with no leveling or hammering down. remember to create a fall<slightly higher at the house>away from the house. it dosent need to be much as rain water will run at almost level
good luck, hope i helped and didnt confuse you.
im a concreter