Hi, this is what I did with Scarlett.
First I hung it in the cage in areas where she didn't spend much time. Then in places where she did (at the end of her favourite perch).
Then I hung it near her food bowl. It took a little while for her to start eating again. When she was comfortable I moved it slightly closer and closer, then draped it over the food bowl so she'd have to stick her neck through to eat.
When she was happy with that, I'd have it round my wrist when I asked her to step up. Then later, round my hand and fingers. I'd also leave it round her t-stand and encouraged her to play with it by giving her a seed if she touched it.
Later, I decided to feed her outside the cage to avoid mess (we started getting ants), and before I fed her I'd put the harness on, she would fuss with it for ages but I'd leave it on until she'd at least taken a bite, then take it off and take away the food. Gradually over two weeks or so she'd eat more and fuss less until eventually she didn't fuss at all! When she was finished eating she'd walk towards me and look at me to ask for the harness to be taken off.