My DD was having it for sleeps and had stopped having it during the day. We waited until we knew we would have a couple of normal days ie we had just moved waited a few weeks after that and had no big appts on, she wasnt sick etc.
I spoke to her about giving her dummys to the other babies cause they would be needing them soon, I mentioned this for a while before we did it just to plant the idea then on the day choosen I asked if she wanted to collect all the dummys from around the house put them in a nice little gift bag and put them in the letter box for the dummy fairy to come and take to the new babies as they now needed them.
She did this and we walked to the lettr box, my sister ran up to the box after we came back in and got them and hid them in the boot of my car for me to secretly dispose of later and put a new princess wand in the letter box. I then said to her lets go see if the dummy fair has been yet and off we went she checked the ltter box and was so excited she got a wand in the mailbox that the dummy fairy had left to say thankyou for all her dummies. We made a big deal out of how she was a big girl letting the babies have her dummys etc and how cool her wand was. We did this early in the day so that she had her afternoon sleep without them and got use to it rather than right before bed.
It was a bit of a challenge getting her to bed she did ask for it but I just kept saying you dont have dummies any more the fairy as taken them to the new babies mummy cant give you one because we dont have any and told her that was so nice of her to give them to the babies and now you have an awesome wand and she was allowed to put her wand on her dutches the first night then it ended up with all her other toys.
So She really went cold turkey but I tried to make it as nice a transition as possible..
Hope it helps, good luck.