Hitting the sweet spot of your putter has three advantages, it comes off the face more consistently, with more energy and has over spin instead of side spin
Hitting the ball with the bottom of the club face can result from several different swing faults
Focus on your left wrist at the top of the backswing and start of the downswing to get to impact properly, many players have a cupped left wrist which changes the angle of the clubface, this causes an open clubface and results in casting and manipulation during the last portion of the downswing