When it comes to skills, the most important one in mountain biking is braking. You will use your brakes a lot more than you shift gears, corner, descend, ascend, etc. Braking also happens during, or before, all other skills. So when I get absoute beginners who want to learn how to mountain bike and do it right, the first real skill I will teach them is always braking.
Most people will use anywhere from 70 to 100 percent of their rear brake when it come to mountain biking, but when it comes to proper braking it should be more like 60 percent (or more) with the front and 40 percent (or less) with the back. I know what you are going to ask. But that's going to throw me over the handlebar like a catapult in the Trojan wars! Well, not if you are in the proper position.
When you need to brake hard and slow down quickly, you need to have your body weight way behind the saddle. Once you are in the proper position then you are safe to use the power of your front brake.
The front brake will always slow you down faster and without losing your line. If you lock up your rear wheel you will lose your line and, of course, it will take longer to slow down to the appropriate speed.
So here is how I would practice this skill: