Converting 5.x FormsAPI code to 6.x

I recently stumbled upon a handbook page here for altering node create forms to make them multistep, however the code provided was written for 5.x. Exactly what changes need to be made to get it to work with 6.x? I'm fairly new to messing around with the new FormsAPI in 6.x (I have done some work with multistep forms in 5.x awhile back) and hooks, so any pointers with regards to the code used in the book page would be appreciated.