I only told the family I routinely talk to (through phone calls). By the time I told them though, they'd kinda guessed what was up anyway, as they're also the ones who saw me the most.
Otherwise, I figured I'd deal with everyone else as I bumped into them. I didn't like the idea of calling up people I rarely talk to just to tell them about this. That seemed kinda... I dunno... presumptuous? I had a rule to not over-explain myself, as at some point it starts to seem like "apologizing" for who we are. Why CAN'T you just walk into Thanksgiving Dinner as you now? Do you really "owe" them a forewarning?
With me, as Fate would have it, not long after I told the immediate family, I ended up photographing a niece's wedding. Meaning... I'd be out in front of the entire extended family... as Kate. Oh my!
I went, and braced myself for having to go through endless explanations... but ya know what? Word apparently had spread, as everyone I met just called me Kate right away. It was the weirdest experience, it was like I'd always been Kate as far as everyone was concerned. The whole transition subject never came up at all, not with anyone. I was apparently just Kate now with them, and that was that.
So I dunno, I'd say "seed" the information with people who are closest to you, and let Fate take care of the rest
~Kate~