Good question. Many people try to hold the strings down on the frets all of the time like you would on a typical guitar strum. DON'T! It's far too much work. The chop works like this: as you are moving the pick towards the strings to strike them, your fingers should be "resting" on the chord you're prepared to play. Before the pick strikes, quickly depress the strings and let up as soon as the pick strikes all of the strings. In other words you're quickly dampening or muting the chord once it is played. Your ability to cut off the ringing sound defines the amount of "pop" sound you want.
Be aggressive! Most people are afraid they'll make too much noise. I'm here to tell you that with a guitar, banjo and bass there's not a chance you'll be too loud. But remember, it's not loudness but "quality" that matters. However, you have a role to contribute and you can't do it unless you're heard.
G, C & D Chop chords. So named because the mandolin serves the purpose similar to a drummer in a bluegrass band by playing a percussive "chop" chord that falls on the 2 & 4 beat. The bass plays on the 1 & 3 beats and combined you've got the basis of an acoustic snare and kick drum!
These three chord "positions" form the basis of literally thousands of songs. They are the universal chord fingerings in that they're moveable up and down the neck and create other chords. Learn these three and you can easily move them up to play in the keys of A, Bb, B and C.>
The "G" chop chord is the hardest chord to learn when you're starting out. The first time I made that chord I looked at the person showing me and said something like, "you're kidding, right?" Well, it's a good starter because once you get that and the four-finger "D" chop chord you've done the hardest chords there are in my opinion.
Play Along with the Recordings
Seem like a stupid thing to do? I get endless requests for "which is the best video," or "what's the best book to buy?" Hey folks, I'm here to tell you there's no substitute for putting on the CD's, tapes or records and trying to play along. It's great for your timing, good for your ears (and hands), and forces you to really LISTEN. Until you get the sounds in your head you'll never be able to play them on your instrument.