My horn has a powder measure attached to it that will measure out the correct charge for my rifle. I wear my bag on my right but carry my horn on my left. I carry my horn on my left for two reasons, first, it is handier for me to handle my horn with my left hand and hold the measure in my right hand. It's just what works for me. Secondly, I am right handed and carrying my horn on my left side keeps it away from any flash or sparks from the pan on my rifle. It's just a safety thing for me.
In use, I set the butt of my rifle on the ground and hold it to my side with my right arm. I reach into my bag and take out a patch and put it into my mouth to wet it while I load. I pull my horn around with my left hand and use my right hand to remove the stopper and hold the measure while I pour powder from my horn with my left hand. When my measure is full, I re-stopper my horn, let it hang while I pour the powder into my rifle. I then release the powder measure and let my horn swing back to my left side whie I reach into my bag with my right hand and remove a ball. I remove the patch from my mouth by sliding it between my lips to remove any excess moisture. I don't want it dripping with spit, just moist. I use the patch and ball to finish loading. I then reach into my bag, remove my priming horn and prime my pan. Close the frizzen and I am ready to shoot. With a bit of practice, it is not a difficult thing to do and can be done rather quickly. It will get you away from the modern "in-line" pre-measured way of doing things and help you discover the pleasure of doing it the old way.
If you wish, you can use a device that is made of wood and has holes in it for pre-patched balls. This is hung around your neck by a lanyard. When you are ready to load, you pour in your powder and then use a short starter to press one of the pre-patched balls out of the wooden piece and directly into the muzzle of your rifle. It is historically correct and will speed up your loading just a few seconds. If you use such a device, you will have to use some sort of grease to lube your patches becasue spit will quickly dry out. Since I prefer to use spit on my patches, I do not use one of these devices but they are handy.