It depends on the call. Some will use the gargling technique and others will flutter the tongue. Either way will take a lot of practice.
I flutter the tongue over the reeds. Tongue pressure is light and my mouth and lips are very soft. Tongue is positioned on the call a little farther out than a yelp. Remember, as always with a mouth call, to push the air over the reeds by exhaling the air from the lungs. Move your tongue in and out to find the spot for you. Slowing down the flutter will give you a softer purr while a faster flutter will sound more like a fighting purr.
It took me years to get it right.