Firstly, can I ask why are you starting to use a harness at 8 months old and not before? Is there a problem with her pulling?
For puppies, harnesses are much better as much less chance of damaging their neck if they pull, and less chance of slipping out when they're so small....
Your puppy sounds like she needs desensitising to the harness, as you haven't used one before so all of this is new to her. Leave it out on the floor, chuck treats when she goes near it so she associates it with good stuff. Pick it up, let her watch you WITH IT, give her treats, then work up to putting it on and treating, then taking it off again.
If she backs off or displays fear you've gone too quickly, so start again, she just needs to build positive associations with the harness.
Puppia do good over the head harnesses and they're very soft too.