Gst registration of proprietorship firm under huf

I have formed a firm where the proprietor is HUF, we have the trade licence in firm name where HUF is licence holder of the same. but I am facing little bit of problem in understanding the documentations, What will the format of authorisation letter? What shall I fill in applicant name? Which stamp shall I use in authorisation letter? HUF or Proprietor?

I request to please help me.... Please let me know the format of letter, stamps to use on it and by which name shall I apply it.