I usually dig a hole in the sand next to a rock and put the anemone there. Either hold it a few minutes until it attaches or turn off the pumps so that it doesn't blow away.
Note: The sand falls back into the hole, you cannot keep it out, the anemone just needs a place to start.
It will use the rock or side of the tank like a brace as it burrows its foot into the sand.
I have a LTA and a 6 inch DSB. It's foot goes to the bottom glass and it is very happy there.
If water flow, lighting, and feeding are satisfacotry, it will stay put. If not, after a few days it will move.
Mine detached and drifted the tank two or three times before it found a spot it was happy with.