I have tried many times with Ben to get a harness on her from about 5 months old and no success, it will take lotts of patients and reward to get her to accept it and to be honnest I gave up first.
Do it bit by bit and often, and I am sure you will get success.
As for outside, get a good one I tried the avaitor and still have it, the video is ok but it is a bit over the top.
SN.