STANCE: Your stance should normally be the narrowest of any shot that you play. (I'd recommend the outside of your feet be no wider than your hips.) Your stance should also be taken near enough the ball so that you can produce a stroke which is straight back from, and straight through to the hole for putts of nominal length, while not so close as to have a tendency to force the clubhead to the outside of your target line on the backswing. (If they dropped a ball from the bridge of their nose, most good putters would want it to land on their target line.) The feet should be turned open, closed, or square, relative to how those adjustments affect your swing's path, with the right foot primarily responsible for your ability to follow-through correctly. Most, although not all, good players address the ball so that their weight is balanced slightly toward the inside of their left heel.
BALL POSITION: I recommend that you play your ball somewhere between the center of your stance and the left instep. This allows both the path and the clubhead to square up to the target prior to impact, and it allows any approach angle to level out enough to promote a good follow through. (Sweeping type strokes are probably better positioned left, while a tapping type action might tend to be positioned more toward the center.)
GRIP: I hate the word grip, and the implications it holds for most people. What you want is to lightly "place" your hands on the club in such a way that it's easy for you to swing the clubhead squarely toward the target. For simplicity's sake I'd recommend using a very light version of your normal grip, with the exception that the little finger of your right hand be on the club rather than overlapped, or interlocked. Be sure that in closing your fingers you don't force the club into some angle of lie, of loft, other than its designed one.
STROKE: Your object in all of this is of course to strike the ball with the clubhead so that it rolls surely and truly to the target. This will be best accomplished by accelerating the clubhead through your point of balance (not your hands, elbows, or the grip of the club), and fine tuning the components of your basics until this acceleration occurs down the intended line, with a clubface which is square to it. 
