Well what you are asking about doing is not really simple.
it requires some experience with databases, creating tables and creating recordsets.
my advice is to take it slowly, and get your head around each part.
create the basic download page first, without worrying about limiting the download to 1 per user.
once the download is working, move on to the next part.
if there is a particular part of the process you have a question about, let us know.