purring tutorial:

1) make a "kuh" sound, just like you normally would pronounce a K
2) now shift the arch if your tongue back a little and do the same thing - it should sound a little squishier now, like "khuh"
3) hold your tongue there and exhale through the gap. adjust the height of your tongue until it vibrates
4) now close your mouth most of the way, just leave a small gap for the air to escape

prrrrrr