For an advanced serve, use a hammer grip. Toss the ball up, pause a bit while you find the ball, addressing it with the racquet’s edge.
Move the racquet up and to the right, pronating your hand. Your index finger, if extended, would point all the way to the right, with the pinky pointing to the sky. Then you would let the racquet and hand come down in front, now leading with the opposite racquet’s edge.
Don’t flatten your serve completely. Always generate some spin.