A local reefer used to propagate BTAs on a huge scale and gave our club a demo. First he inspected the foot of the anemone; apparently this is where most of the energy reserves are, so the fatter the foot the better. Then he dunked it in the water a few times to get a better view of the mouth. Then he looked closely at the mouth, they're a slight oval shape - cut it along the long way, it'll give it more mouth to go from. He used a new razor blade and slid it from the mouth on one side then turned it around and did the other side. He keeps them in high flow so that fungus doesn't form and kill the anemones. Also try and raise your mag, that'll help keep the fungus at bay as well. These are all based solely on what he explained to our club and I cannot back the claims except to say that he's done this literally thousands of times before. HTH.
Cayman