Some salient points (given the fact that you were double handed before).
-- You must adjust your grip to full Eastern BH grip;
-- Left arm pulls the racket back and up causing upper body turn from square stance position (you get more upper body rotation with 1-handed BH).
-- Both shoulders turn -- but the front shoulder should not dip -- they should be on equal plane! Turning of the shoulders and dipping the shoulders are two different things. Turning is good, dipping is bad!
-- In the backswing, the hitting arm is slightly bent -- elbow-bone pointing to ground.
-- As the forward swings starts, the left arm and left foot stays back!
-- Just prior to contact, during contact, and after contact the hitting arm is stiff and straight making 90 degree angle L with the racket (you must not lead with the elbow; this occurs when the hitting arm is loose).
-- You stay sideways during the hit (as opposed to facing the net with the double handed BH).
I think the above will help.