Add fields to account

I am building a module for drupal6.
this module has to add some form fields to the user accounts.
so when i register/edit an acount i get the form that i build in the module under the default fields that are already there.

I searched the code of the profile module that does exacly that, but could not figer out how they did it.
Can someone please point me in the right direction, all help is appreciated