Hitting the ball with the bottom of the club face can result from several different swing faults
A common mistake on the backswing is to hinge instead of cock your wrists, a perfect drill to help fix this is to pre set your backswing by cocking your wrists right up in front of your face at address and then turning into a full backswing
Choke down on the club slightly, keep the ball back in your stance, place 60% of your weight forward and finish low