Well, different people use slightly different approaches.
Mine is basically this (three-finger clawhammer). Fret chords with your left hand as normal.
Right hand, your first, second and third fingers are reserved for the G, B and E strings respectively.
Your right thumb can pick any of the three bass strings as appropriate.
So you might play a D chord, and play a simple um-chug pattern with an alternating bass by plucking the open D string with your thumb, followed by plucking all three treble strings simultaneously with your three fingers, followed by plucking the open A string, then the three trebles again, then open D, and so on.
Which string your thumb hits depends on which string the bass and fifth of the chord occur on.
Moving on, you might play something more sophisticated than just all three treble strings together, such as Thumb, 1 2 1 3 1 2 1 and repeat.
As you get more advanced, you will learn to fit in runs with your thumb, play bass notes on the G string occasionally, bring in your right hand little finger and so on.
I'm sure there are videos and other teaching material on line, and of course you can't beat a few sessions with a real live teacher.
Handy hint: keep your left hand nails short; let your right hand nails grow a little to give you a clean, bright sound. Try not to get in to the habit of resting the heel of your right hand on the bridge or soundboard as this cramps your technnique and damps the sound of the guitar.