First, since Flourite is coarse and therefore prone to allowing sand to filter into it,
you would be better off if you used a bit larger material than say pool filter sand.
I use fine gravel which is actually #4 blasting sand(yellow pages/sand blasting/40lb
sack $12) but Mom & Pop type shops carry it at times for substrate. They just buy the large bags and break it down onto 2-5 lb bags. This is the size of it and it would be far less likely to dissapear into the Flourite.
http://www.plantedtank.net/forums/picture.php?albumid=14281&pictureid=52570
But if you want something about as large but in white you might look up Petco "Crystal River" gravel/sub. at Petco on-line.
I'm sure a person/w experience at it can do better but I'd just use a piece of garden hose and vaccuum a trench where I wanted the path to be and then fill it/w the sand.
You wouldn't need to go deep. Just a depression in the surface. You could get a piece
of PVC(small diameter) and a funnel and use it to get the sand in the trench instead of all over. Just stick the funnel into the top of the PVC and slowly pour the sand as you
direct the pipe along the path. Probably better if you had a "helper" to pour the sand
slowly out of a cup.