Here a forum that deals with the same topic your wanting to learn about:
http://www.ukuleleunderground.com/forum/showthread.php?t=164
As for advice. . .uhh I just strum down with my finger, but before my finger makes contact with the first string, my palm is already on all four strings and mutting them.
So I guess to break it down:
1. you strum like you normally do
2. on the strum you wanna do the crunk sound, try hit the strings with the lower left of your palm first (the outside part under your pinkie finger)
3. while simutaniously strumming with your finger(s)
4. once you finish the strum you can lay your hand flat on the strings to maybe add to the percussion effect. . .
I'm not sure if that makes any kinda sense what so ever, but that's how I do it. I'll try and make a vid in the near future to further explain it to you if that'll help.
Ne ways, hope that helped, if you gots any other questions feel free to hit me up
