Trial and error, mostly. If you zoom all the way in on the round extender, you'll notice it's not exactly round, more like octagonal (eight sided). You have to first place a piece, then touch another piece to the side to find out which direction the attachment point faces. Delete the one you first placed, create another and set it in place, but DON'T attach it yet. Do you remember what direction the attachment point was facing? Okay, now rotate your piece, all the time watching where your attachment place was, until you get the point set facing the direction you want it to be. Now click and attach your piece. check the attachment point with another piece as noted above - if it's not in the right place, delete it and try again. it takes time to learn, but this is how all the top builders do it.
