I don't have a pro account on Railscasts either, but sometimes it is a good idea to have a look at Ryan's Github account. Oftentimes he develops gems to stuff he covered in his episodes. There you will find this awesome nested_form gem, which does exactly what you want.
Of course you can reinvent the wheel, but I think it is much easier and faster to use Ryan's gem. The documentation explains perfectly how to use it. Have fun!