Poor posture leads to your body parts not being in the correct position to rotate around your spine. This leads to over-rotation, where your body rotates all at once instead of as correctly needed in the swing.
The best way to read greens is through experience, I like to take eight balls and roll them to a hole at different speeds and on different lines, then with the eighth ball I putt it by picking the best speed and line to get the ball in the hole
Use this simple tool to help you square your putter face to the hole so that you can hit putts in the direction you aim