On the top - make a lemming climb and bomb. Build to let the lemmings come out and get up.
On the bottom - go through the tunnel, then start building up. I think you build as soon as you come out of the tunnel, but it might be as soon as you turn around after coming out. Either way, keep building until the next builder would hit his head on the roof. Have the lemmings up the top make a hole in this roof - I think you use a miner, but it might be a bomber. You have to position this hole so that you can build through it. If done right, all the lemmings will now be together up the top(or walking there).
I can't remember the rest of the level, but I'm pretty sure it's just one turner somewhere.
This, however, is one of the levels I can't remember very well, so I might be way off. I'll play the game now and see.