How do two girls began a threesome?! Urgent Please!!?

Threesome happening soon. Two girls[one me] and my bf. I want it to like just happen. But I want to start it. And I dont know exactly how to go about starting it. I dont want anyone saying how bad or gross this is. I know everything about it and its well planned out. Just would like some help on how a girl[me] can start this..Thanks!;D