Here's the way I was taught,
1. Relaxed focal distance is about 5' so constant movement and deliberate 'looking out' is required so that you don't just relax and see 5' in front of you.
2. You will pick up relative movement, more likely with periferal vision rather than direct vision. So you have to pause in your lookout sometimes so that your brain gets a chance to pick up an objects movement in relation to the background.
So the actual method, or one way,
.............. is start at far, lower left of screen and move your eyes from left to right and up and down in a sine wave shape. Each time your eye reaches the horizon pause and give your brain a chance to pick up any objects moving. Each time you eye reaches the top or the bottom of the movement pause and again give your brain a chance to see. You will finds that this gives an efficient lookout in a very short time.
It's important to remember that you will sometimes pick up objects as they move in relation to the background and other times when your eyes rest on them when you are 'searching' for objects.
Finally teach them a reasonable way to point out aircraft that may be conflicting ..................with the clock code + expansion ie aircraft left 10 o clock low.............or helicopter right 1 o clock high etc etc.
Hope that helps,