Hi Natty,
Nature is amazing in the sense it has given the human species either gonads or ovaries. This is responsible for Testosterone or Eostrogen. It's much more complicated than that so let's just move on...
So you want to introduce the sex hormone of the opposite sex...
Well there are a number of ways, injection, oral (tablet) or patch.. Any others girls/guys?
So the first step if your wanting to seek HRT is to speak with a Psychologist or gender Cousillor. From there you will be referred to an Endocrynologist whom can prescribe hormone replacement therapy for you.
Hope this helps answer your question