A mistake I see many beginner-intermediate riders make is gripping both handlebar grips with all 5 fingers and only grabbing the brake or clutch when needed.

The problem with this is that you loose control of your bike during that transition from grips to clutch/front brake lever, and if you’re doing it while riding through braking bumps, you have a high chance of loosing control of your bike or having your hand slip of the grips.

You need to train yourself to ride with 1 finger on each lever, so that you’re always ready to use them right away while still keeping complete control over your bike.
Which finger you use doesn’t matter, as long as you’re comfortable doing it. I myself always use my index finger on both levers, but many riders prefer to use their middle finger to clutch and brake and keep their index finger on the grip.
I tried it a few times and couldn’t stand it, so it’s really up to you. The image below shows Christophe Pourcel using both his left and right middle finger to grab the clutch and front brake lever:
Let me clarify something: you don’t need to always ride with 1 finger on the clutch or front brake lever.So how do you know when and where to do it or not?
Well for example, a good place not to do it would be going up a hill or going down a fast straight, as you don’t need to be ready to brake or use the clutch since you’re wide open, but you need to make sure you get ready in time for the upcoming corner.
For example, if your going down a long straight wide open, and there’s a tight right hand turn at the end, don’t wait until you’re in the braking zone to grab your front brake lever and clutch! Get ready a few seconds before by moving both fingers in position (you can do this while still being wide open), so that you’ll be ready to act before hitting the braking bumps, not as you ride through them.
Now get off the computer and go practice!