In my experience these are some of the most difficult anemones to get to attach. Definitely dig a hole for it, preferably next to some rock structure, but don’t cover its base completely with sand as in my experience they don’t like it when attaching and will inflate themselves to rise above the sand. Putting some rock rubble around the area that’s exposed to the sand is also a good idea. Keep your flow on the low side, but not completely off, for a couple of hours to allow it to attach. If it’s healthy, it will attach in a couple of hours. The hard part is keeping it attached. Any change in flow or lighting, and I mean ANY, and it is likely to detach in a few days or a week to try and find a new home. If it’s happy, it will stay there hopefully for a long time. Good luck!