To make the tree for ordinal α, start with α at the top, then do this process to α, then do the process to the members of α, then do the process to the members of those members, and continue repeating.
1. If ordinal β is a sequence ordinal, nothing happens to β.
2. If ordinal β is a limit ordinal, the children of β will be the first n members of the FS of β. n is called the base.
Also, I am redefining HN. You can check the HN definitions if you go to my site, there is a link in my userpage.