From someone who not only knows how to do it, but who knows how to teach it.
You can just hop on a bike and pick up quite a bit on your own. But you’ll undoubtedly pick up some bad habits. You’ll soon hit a plateau, as those habits keep you from advancing. If you’ve ever taken ski lessons, you know that a few well-placed words from an instructor can quickly help you learn a new technique. It’s the same with mountain biking.
You can also improve your technique by biking close to a more skilled companion, and watching his or her moves carefully.