Use the FRONT brake. 1) It’s linked to the back anyway 2) When you brake, the weight is transferred forward giving more grip to the front and less to the back, so the back is more likely to lock. DO NOT GRAB the brake, squeeeeeze it progressively. The initial light braking will load up the front suspension and front tyre and you can smoothly increase the braking force. Look up Motojitsu on YouTube - lots of good stuff to practice.