One more note to add: rather than using the little shelf holder pegs that came with the bookshelf, Casey bought the L-shaped kind so they could screw in and the shelves won't flop around and the kids can't move them.
That's it! It can be as easy or as time-consuming as you want it to be! With the furniture I scored on Black Friday, our total was less than $60. The nice wooden Barbie houses and even most of the plastic ones are much more than that!