The discussion here applies only to the United States.
If you are building a balloon in another country, drop us
a line and we can point you at the appropriate resources.
An envelope built accouding to these plans
is not likely to be
light-weight enough to be an "ultra-light vehicle."
Therefore, you will need to register it and obtain
an airworthiness certificate from the FAA. That
always sounds a bit scary at first. But, with a little
planning, the regulatory process is fairly straightforward.
First, we recommend joining the
Experimental Aircraft Association (EAA).
In addition to helping your karma, the association provides its
members with free techical assistance via telephone. So you
once you are a member, you can call the EAA and ask knowledgable
folks about FAA rules and procedures.
Second, we strongly recommend that you purchase the EAA's
Amateur-Built Aircraft Certification Kit. The kit contains all of the
forms that you will need (some of which can not be downloaded)
as well as a booklet that gives step-by-step details
about filling out the forms and other regulatory steps.
Another thing to keep in mind is that the FAA requires
that you keep a log of all of your time spent building your envelope.
You should also take lots of photos of yourself doing the
actual construction work. The FAA will want to see the log
and photos as proof that you did indeed build the "major portion"
of the aircraft.