mo wrote:
I bought 2 large T hinges

Good, that's a start.

Whats the best way to fit them?

With screws - preferably rust-resistant ones - and don't forget to fit a
gate catch as well!

Onto the post first or onto the door first?

Generally you fit them on the "door" first and then the gate jamb.

I have left a bit of space on either side of the door to fit between
the posts.

Now that's a good idea, that'll save you having to initially plane them to
fit - and then it also allows room for the 'door' to swell during rainy
weather.

Mo, I seem to have missed something here, is this an extra to a post to one
that you have made about making a garden gate and want advice to fit it...or
something else?

Now presuming that it's about fitting a T&G garden gate, then make sure that
the hinges are screwed on the ledges (the horizontal bits) and that any
braces (the angled bits) run upwards from the hanging side to the slamming
side to take the downward force of the front of the gate and prevent it
dragging the ground after a while.

Now feel free to give any more information that would get you a definitive
answer

Cash