once the water bags have burst or are hanging out,
That's it, until you see that then she isn't lambing. Once she is, then you do need to find out what's going on if nothing appears after half an hour or so.
If you do have to 'go in' you just have to imagine that they're supposed to come out two front feet and the head, lined up as though they were diving. If the 'head is back' or one leg, they'll get stuck. You gently (with short finger nails!) push the lamb back in, find the relevant missing bit and line it all up. The lamb will now come out. Usually.
They can be totally backwards of course, in which case you just have to pull them out backwards, but quite quickly. Trace legs back from the head, so you don't mix up one twin's head with the other one's legs! Doesn't happen often, usually one twin is completely behind the other.
Occasionally they're lined up right but they're big and in a first-timer, you might have to pull on the legs and then the head behind the ears, once it appears, to help.
If you have to intervene, always give the ewe an antibiotic jag afterwards.
Sorry if this is egg-sucking advice