In this video, PGA Teaching Professional Todd Kolb teaches you how to read putting greens. The first thing you need to do is to look at the green as a whole. Most greens will have a general shape to them. Some will have more undulation than others, but you need to recognize the slope or the angle at which they sit, meaning back to front or front to back.
The next tip is to actually walk to the distance of your putt. So, you start at your golf ball and walk to the cup. A couple of things are going to happen here. This will help us feel the distance at which the putt actually is. While doing this, use your feet to feel how the putting green is moving (uphill, downhill, or side hill). Lastly, after gathering this information you need to pick out a target at which you want to start the golf ball and get it going on its intended line.