The Half Keyboard is always ready. You type with your left hand, just like you would on a normal keyboard. Except, the spacebar also works like a shift key. Holding it down gets you the letters of the missing half of the keyboard. Hitting the spacebar alone still types a space.
Most touch typists can learn it after only a minute or two of experimentation.
Very little effort is required, and speed increases rapidly with practice. It's no easier and no harder, and will take roughly the same amount of time. The finger used to type each letter is the same finger that's used in normal touch typing. That's why you can learn it so quickly. You're using the same finger movements that you already learned in typing class, except now, you're only using one hand to do them.