To begin your c-tap, run in the desired direction and press crouch for long enough to have your view shift slightly lower, that way you have your speed maintained but you are technically in a crouched possition.
Try just doing a simple jog and tap the crouch key just so that your view shifts slightly lower but make sure your move speed is maintained constant
After your view shifts simply press jump, while continuing to hold your crouch key, and shoot a rocket at your feet just like you would with a normal rocket jump.

How is this different from a normal c-tap you ask?
You dont have to strain your pinky while trying to double tap that crouch key. (Yay)
This can be also applied to sticky jumping and grenade jumping for the demolitions man.

Lack of images due to me being a lazy skrub
(Srry :S)