Some people think it begins when you swallow your first estrogen tablets or take your first testosterone shot and ends when you have some kind of surgical intervention or the morphing into the body that matches your brain gender map is complete.
But I submit that's not the case. Transition may start when you take that first estrogen or testosterone shot, but the endpoint is when they close the coffin lid on you.
The middle between those two points is the life you live and how you define it.
Like life itself, transition is a constantly evolving series of events that are part of the totality of your life until it's done. It is a just battle for the self determination of your own identity to paraphrase what my little sis Jordana has eloquently said about it. It is a long journey of self discovery that at times can be a pain in the ass. There are periods of introspection and analysis you'll have to go through in terms of whether you're living up to the goals you set for yourself when you began your transition. Sometime you do that hard solid thinking that comes with that honest assessment of where you are in terms of your transition goals on your own, sometimes it's a group exercise in concert with others.
My personal guiding transition principle and has been since 1994 was wanting to be a compliment to Black womanhood, not a detriment to it.
When I sit down and do hard solid thinking on where I am in terms of reaching this goal, it is with this guiding principle in mind among many others.
But as you're on this journey, you want to strive to live a quality life an which you are happy in your own skin. How you define that happiness and what a quality life means is up to you.