If you think of the chakma as a smooth butcher's steel. you won't be far off.
It moves metal, it doesn't grind it away. If it has been properly hardened, it will align and true the edge, and application to a dent or ding will force the metal (or most of it) to flow back where it belongs, so a lot of grinding isn't needed. The thin sides or even a corner can be used to apply more concentrated localized pressure where a ding or dent is. Just work the blade in sections since the chakma is so short. Work the blade with the flat of the chakma in over lapping sections using light pressure to finish. What can move metal where it's needed to align the edge can also move metal out of alignment especially if one gets frustrated, or uses the corner of the tool.
Sometimes the chakmas aren't hard enough to do the job on the khuk's belly, since another use of them used to be to strike sparks with flint (cig. lighters have been in Nepal for a while). The shaft of a large screwdriver or a smooth butcher's steel can be used instead, and may be safer to learn with.
It's to maintain an edge, not put one on. If the edge isn't set, one will need to be put on with some kind of abasive method. Try to use it on the karda first--the kardas are usually a bit softer than the khuk's belly. Or try it on a small kitchen knife. And if the khuk's belly deeply scratches the chakma, the chakma may be too soft--but it will still work on the softer tip and recurve of the khuk.
Don't push so hard that you lose control and slip, and pay careful attention to what actually happens to a decent edge when used, and you'll get it. Like other hand sharpening methods, one just has to get the feel for it. So be patient at first, and work safely.
Hope there's something useful here--like with benchstones, there's not one correct motion to use, everyone has to find what works for them.
A grooved butcher's steel will probably work well on the softer parts of the khuk, but again these aren't intended to make an edge but to maintain one. The grooves on the steel act like little corners to apply strong localized pressure. It's not a file. A grooved butchers steel can actually chip out a hard brittle edge like some kitchen knives have.
If you can get the edge into a V-sharpener, you may eventually change to knife's profile to match the sharpener and get a sharp edge. I'd rather keep the orginal profile myself and live with a few scratches on the khuk than change the intended edge to the profile of a sharpening device, especially a device that is often intended to sharpen tools for skinning or food preparation. A V-tool intended to sharpen axes may work OK, but I'd still rather try and work towards a nice polished convex edge on a khuk. Tape up the blade flats if potential scratches from slips are thought to be a big issue.
