Placing your thumb on the side of the fingerboard is good! Generally place it toward the bottom of the fingerboard and on the side, not underneath. Pluck over the fingerboard, toward the end of it. Any lower (between the bridge and fingerboard)gets awkward and if ever using a bow will get rosin on you. Use the SIDE of your first finger. This pulls the maximum amount of tone from the instrument. Use the weight of your arm to fall and pull the string. Keeping the finger / hand / arm RELAXED (of course). It should be one motion with the arm and everything. Don’t use just the base joint of the finger, although it should be relaxed and may bend, just don’t initiate the pluck from that joint. This will use the greatest amount of weight possible for volume and after practice speed will not be an issue. Most students think that 2 fingers is optimal, and for some extremely fast pieces this is used, but 1 finger used this way will get a great sound.
Think of the arm as falling (gravity pulling it) down and bringing the finger with it which pulls the string.
This concept is much easier than it sounds!
Jazz Plucking Example 1
Jazz Plucking Example 2