You'll have to practice, there's no way around that.

When learning to purr with a mouth call, try one that with reeds that are straight across the front. Notches and V-cuts in te latex will make it harder to get a cleaner tone.

To purr with a slate call, you'll probably have better luck with a surface that is rough and doesn't need conditioning a lot. The other part is finding a striker that works for you. You'll find that purring on a slate is about finding the right pressure. Surfaces that will generally purr well are slate, ceramic-like sla tek, some of the ploycarbomate calls (power crystal). I have a glass call that I like, but it only purrs well with certain strikers. If your striker is too soft, it won't produce a clear tone.

Box calls...just the right touch, and you can get a really sweet purr out of it.