If your locking at the elbow joint, or even allowing a slight break, you might have a problem with the shoulders. Taking more of a bend at the elbow might help, and you really don't have to have the thumbs almost pointing to the floor and the little finger up. Gironda's methods did not always work for everyone and can be down right painful for some. We each have a different bone/joint leverage type. With the lateral raise, you really want to lead with the elbows and not the DB's themselves.
Starting the DB laterals from behind the back, rather from the front of the body as most do, will have a different affect on the delts. If standing, than the legs will have to be bent a little. Some do these sitting on a flat bench or a steep incline bench, with good results. Remembering to keep the arc of DB's behind you as you begin the movement. Good Luck.