Two methods are quite easy to learn. The first, involves less risk. Grip the knife fully and reach with you thumb to depress the lock. Press the spine against a part of your body, say your leg or side to slide the blade halfway closed, then move your fingers out of the path of the blade and slide your thumb over the spine of the blade to close.
The second is pretty much the same thing only you have to make certain you remind yourself to have your index finger all the way forward. Press the lock with your thumb and flip the knife to cause the base of the knife blade or choil to close on your index finger, then slide your thumb over the blade to close it fully. This is only recommended when you have an unsharpened section at the base of your folder.
Believe me, I'm not a knife nut, and I learned these moves pretty easily.