If it was me offering guidance to anyone operating a knuckle boom in your field. I would first agree on what the different boom components or sections are called. Swing, Main boom, Outer boom, Outer boon extensions, Fly jib, Fly jib extensions. Something along those lines. I would suggest using a good head set so both the crane operator and the person giving the commands clearly hear each other.
Here is the hand signals used in the NCCCO training manual page 54
Those signals are standard crane signals, I agree discuss it before hand.
In the past i've tapped my elbow to indicate I was signaling for the jib. According to the standard crane signals tapping your elbow is for the aux. winch, since most kbooms don't even have 1 winch I think it translates well.
Years ago I probably would have done the same, until I was signaling to an inexperienced kboom operator, and had to stand there for a second thinking about how I was going to get him to do what I wanted.
I have used the standard signals for boom up and down and extend or retract. The only difference for the fly jib being raising the signal hand over the head for the jib and at normal height for the 2nd boom.