How do I position my legs in a triangle?

Hi guys,
Sorry if this is a noob question (I'm sure it is :redface:), but how do you determine which leg goes across the back of a person's neck and which leg "locks" into position the choke? Also, does the leg which supports the one cutting across my partner's back have to be on top or underneath it?

Its just that in class we have been doing the triangle from closed guard and spider guard as well as breaking the grip of the locked hands of an opponent when attempting an armbar from side control by locking your legs into a triangle and I'm really confused as to which leg goes where...

Any help please?

Thanks.