I would suggest also that you start off slow.
Take her outside for enough time for her to experience it (maybe 15 minutes) but it also extremely extremely important WHERE you take for those fifteen minutes.
My 13 year old TAG just this summer started wearing her harness. The first time she let me do it---we went outside on the front porch and sat under the awning. I didn't bring her anywhere, she got to sit still, see a relatively unchanging environment and just soak it all in. The second time we went out on the back porch and sat in the sun---again, calm quiet place. The third time we went for an early morning walk to get her used to moving within the outdoors. The fourth time (and this was a big leap) we took her in the car---this we discovered is actually is easier for her than walking outside as she feels safe and 'covered' so to speak in the car.
Just a little while ago we took her into the pet store with us---she seemed to have fun.
The idea is to start them off slow and then push them bit by bit.
The result is that although my grey is NOT crazy about putting the harness on---she tolerates it---because she knows it means she gets to go outside/in the car--and usually the whole time we're out she's beak-grinding, slow-stretching and tail-wagging (I get smacked in the face with wings and buttfeathers

). She even wants scritches sometimes.
Good luck
