It is impossible to keep it from mixing...
once you put in all your Flourite ...only add a little sand to it and work it into flourite and repeat until you have no air bubbles in substrate.
If you do not work the sand into flourite first you will get " sinkholes " later down the road as it settles.
Then just add however much more sand you want for looks over the settled layer you just created.
I add sand here and there and have also pythoned some right out the door to my lawn in places that I had to much...
The bottom feeders love the sand...my cory will vaccum this stuff like no tomorrow...he sucks the sand in and blows it back out through his gills ... very kool to watch.