Best way, is to due to many proofs.
Try to revisit earlier mathematical content. This is great for two reasons.
(1)Great review of older material. Helps with the eventual GRE. You will find out how much information was not understood the first time.
(2)Since the material is something you are familiar with, you will have easier time reading, and
writing proofs of something known.
I learned to read and write proofs from: Reading all of my textbooks throughout my education. Engage the textbook!
I re learned geometry from: Geometry: Moise/Downs, restudied Linear Algebra from Anton( reading, doing, and doing the proofs on my own). I later moved into more advance material.
I cannot stress the trying and doing proofs enough.
The explanation that Moise gives for Existence and Uniqueness is very GOOD. It was something I did not grasp until reading this basic, but good book.