You would inject into the mouth so it closes in after you inject. Although, IMO, injection methods also including hot water, lemon juice, or even red sea's aiptasia-x aren't very effective. They work a little, but it just seems to **** them off and they come back worse. An aiptasia filefish works pretty well with high success rate (A chance they may nip at corals too.) Some people have mixed success with other natural methods like copperband butterflys and peppermint shrimp as well. There's also a gadget called a majano wand that supposedly turns the cells in the anemone into hydrogen causing it to disintegrate rapidly. Never used it personally, but the concept behind it is kinda cool. Although, it's expensive at about $130 or so.