My first repair was replacing a couple balls on my Flipper Pool. Just take your time and look at the others...I'm no rocket scientist - if I did it, you can do it. I made a spring hook so I could reach springs through a small opening and hook them where I was supposed to - that made life easier.
Pinhead52 did a post on this a few years ago:
https://pinside.com/pinball/forum/topic/mounting-bank-a-ball-balls