There's a book you can get: "The Billings Method" by Dr Evelyn Billings that goes through what to do and how to do it.

You assess CM by observation and sensation, rather than by checking inside. So it's about sensations and appearance of mucus, rather than feeling inside. Vaginas are usually moist, so this can confuse matters if you check inside. There are many "rules" to using the method which are fairly intitive and really make sense once you know what they are, so maybe get the book and have a read through - that will explain the basics. Or, the course costs $145 (it may have gone up slightly) if you wanted to do it. Most of it correspondence.