Let me add one more bit of advice. Bring your rod back to about the 1:00 position and wait. Let that "D Loop" form. Then push your hand forward and snap that wrist and STOP at 11:00. If you stop that roll cast at about 11:00 you can throw the loop in the air and lay your leader out straight.

If you go all the way to 9 or 10 o'clock your line will pile up in a pile. Also, (and you won't see this with a grass leader but you'll sure notice it on the water) If you stop your rod too low then you'll "zing" the line on the water spooking every fish in the pool.

Short, powerful strokes that stop on a dime are the key.

And practice. Lot's of practice.