Of course I am biased, but I think spinning cotton on the charkha is the easiest way to learn how to spin. First, the wheel is horizontal, so nothing happens without you. And everything is supported, so there is nothing to drop. The charkha is much faster than a treadle wheel, which allows the tool to do the work while you can relax into a calm and leisurely meditation.
The ratio of most charkhas is between 65:1 and 80:1. This is considerably faster than most treadle wheels, and about twice as fast as a great wheel. It is about the same as a great wheel with a Minor's head. This is what allows the tool to do the work. It is importantboth to develop the rhythm and develop the touch; grip is undesirable in drafting the fiber. If you have only spun on a treadle wheel, there are vast differences between a flyer and bobbin and a spindle. First, there is no brake and no draw-in. The adjustments are only in the spinner, not the wheel. Winding on is a conscious act of feeding the yarn onto the spindle at exactly the rate that the spindle is turning; the right, turning, hand and the left, feeding, hand must work in concert.
You can purchase either a download or a physical DVD with exercises that will help develop this rhythm here. The exercises are in Part 1.
The shortest answer is that you don't, you use a different spindle or wheel. However, keep reading.
One way to ply is to use the Navajo 3-ply with a drop spindle, not the charkha. Here is a nifty method I call Global Fusion, or A New Spin on Globalization.
Another answer is, in two steps if you are using the charkha.
Step 1: wind your yarns together first into a center pull ball, called doubling (or tripling, etc.) then Step 2: hold the ball in your hand and ply onto the spindle.
Tension the yarn with your fingers as it exits the center, and use the same motions as spinning, except the wheel should go in the opposite direction.
The traditional method of winding your yarns side by side, under the same tension, into a center-pull ball is an extra step that is not necessary for a wheel with a flyer and bobbin. It is still necessary for a spindle wheel. Today you can use a ball winder or a hand-held nostepinne. The yarns are already side by side, you just need to twist them.
Tension is important -- imagine if you tried to use a lazy kate with a charkha. Your arm motions will at first pull the yarn from the spindles, then as you wind on, the yarn coming from the spindles will go slack. They will fold back on themselves, and in all probability you will need to stop and use both hands to regain control of the situation. Hence the nostepinne--take the extra step to avoid headaches and bad tempers. Proper preparation is 90% of success.
A third way is to use a treadle wheel and a lazy kate. Consider that Kate was probably called "lazy" for realizing that with the advent of the flyer and bobbin she did not need to use the traditional method of winding the yarns she wanted to ply on a nostepinne. Since she had both hands dedicated to her yarn and the uptake was automatic, she was able to control the tension "in real time", so she eliminated an unnecessary step. So if you have a treadle wheel, you can just stick your spindles into the sides of a shoebox and go.