You need to inject the lemon juice with a syringe in the base of the anemone. It is a little tricky at first as they will shrink away from the syringe and the lemon juice. I belive they shrink away from the lemon juice on their own because of the acid. I didn't know where to get syringes, so I went by the pharmacy at Walmart. I don't know much about needles so I explained my needs to the pharmacist. After a puzzled look, they suggested I use needles for diabetics as they are very thin. I think it was about $1.50 for a bag of 10. You can probably get them at any pharmacy.
I read this method in an online thread a while back. It worked for me (I had 3 big aiptasia in a 24 gallon tank). I remeber in the thread there was some concern for the extra acid in the tank, but since you are still cycling it shouldn't matter much at all. If someone else is reading this and wants to try it, don't go crazy with the lemon juice and it should be fine. And as always if you do go crazy, do a water change to dilute the acid.