The following exercises are based on the below diagram for hand and leg position:
For a forward lateral flexion (asking the horse to bend his head to the left while moving in a straight line forward) to the left the left hand should bend the head to the side and then lift towards #0. By lifting toward 0, you are asking the horse to lift his shoulder, which will keep him moving forward instead of turning left.
For a forward lateral flexion to the left the left leg should move to position #1 and hold the horses shoulder straight, so he does not turn to the left. This will apply pressure right behind his leg keeping him going in a straight line.
For a forward lateral flexion to the left the right hand should move to position #3 and use just a little pressure to get the horse to tuck his nose to keep him collected.
For a forward lateral flexion to the left the right leg should be about position R2 and help to urge the horse forward
For a forward lateral flexion to the left the body should be straight ahead because that is the direction you want the horse to go