How to ensure a good appraisal and refinance

[Chase Webster]

I understand the value of leverage but am trying to figure out how to plan to make it work for a flip or a brrrr. I have heard that often banks will appraise a property at the purchase price despite rehabs or a low purchase price. How do you avoid this and ensure a good appraisal in order to be able to pull out enough to cover initial costs? Also as far as price point goes, what is the best way to estimate a good purchase price and balance it against rehabs? What is the best way to estimate rehabs? What if I go over budget?