Step 2) For example, let's say we are using ricochet shot. So we must press 1, and after a split second, you hold left click so as if you were charging your rapid fire.
Step 3) Aim for the target. As you aim the ricochet shot, the rapid fire will also charge.
Step 4) Release the left click button, and do tons of damage.
*You do not need to hold the SP skill that you use, you only need to hold the left click button to charge the rapid fire.
I think this works with all of Kai's SP skills, but I haven't tested it yet.
I used the ricochet shot + rapid fire combo in Keaghan and did 47% in a party of 4.
Some visual examples from YouTube: