PE Mysid and depending on the size of the anemone silversides. Silversides can range from small 1/4" square cubes to the whole fish. Feed the anemone one piece at a time, wait until the piece is fully consumed and the tentacles are back open then give another piece. Continue this until the anemone spits out the last piece given. Feed between 1 and 3 times a week.
This will lead to a happy healthy anemone.
If you want to try to keep it in one place associate the place with being a good food source by feeding more heavily and often this, in my experience with BTA's will keep them somewhat in the same spot even if it isn't exactly what they like.
Also be prepared for a large anemone. you don't specify what kind you are thinking about but I had my most recent BTA get up to over 12" in diameter in about a year with 3 feeds a week of silversides. I also had her split 3 times in that timeline because of the feedings which was a nice bit of change for me since she was a rose with green center and white highlights.
Here are photos of her. She was a rescue. I don't recommend this option as the success rate is very low and recovery can be very time consuming and allot of work.
When she came in:
Going into the main tank after spending time in a hospital tank. This method can work to keep them initially in the spot you want them *IF* you make sure there are no gaps they can squeeze of out!
Her right before I took down the tank:
She wasn't fully open here but still about 9" or so.
FTS right before I took the tank down due to a major remodel in the house. She and the tank went to a good home with a marine biologist friend of mine. She took up that whole corner and I had a real problem with the acans next to her eating her.