Formula tips and tricks

Looking for tips on feeding baby formula. I was going to start the transition a month ago but held off. I'm thinking it's time. But I know nothing about formula. Do you use filtered water or special bought water? How much should I start with for a bottle - 4 oz? If she doesn't finish it all, is it still good to save? Anything else that's good to know? I plan on doing this gradually. Only replacing one feeding with a bottle for a couple days. Then two feedings, etc. Thanks, ladies!