Rest your palm on the strings near the bridge, and play as you would normaly do.

The more you go away from the bridge , the heavier the muting ... The more you go near the bridge, the less muting you get.

Check out any Thrash & Speed Metal ... That's where you'll find too many palm muting riffs to practice.

An easy example would be Metallica's "Seek & Destroy" ... A harder one would be "Master of Puppets".