Pull-offs are performed by picking the 1st note and "pulling-off" to sound the 2nd note without picking it. Both fingers are to be placed on the notes to be sounded. Strike the 1st (higher) note, then sound the lower note by pulling the finger off the higher note while keeping the lower note fretted. It's hard to get it to sound right at first, but keep practicing.
In tablature a pull-off is represented by a "P" and a curved line connecting the two notes like in the example below:
Here are some more advanced pull-offs to practice. They're a little tough at first, but as with everything else, it's nothing practice won't cure.
