The roll casts is done in two steps mostly:
1. Move the rod into the D position
2. Forward cast
About 1.: Depending on how far you want to roll cast your fly, you can vary the D (stop) position of your rod between (ca.) 1 and 2:30. 1 matches a smaller arc and a downward tilted forward trajectory (usually best for short distances). 2:30 matches a wider arc and a more horizontal (or even slighty upward) trajectory (best for longer distances). Make sure to keep the anchor + D + rod as good in one (almost vertical) plane as possible. Your rod hand should be positioned in the shoulder to eye level hight. How far you position it (the rod hand) backwards will determine the available length of your next forward stroke. Important: The longer your next (forward) stroke will be, the bigger the D can be at the moment of max rod tip speed by positioning the (main) rotation as close to the end of the stroke as possible. The bigger the D will be, the more forward momentum your line will have. Thus it will be easier to roll out distance.
About 2.: Make your forward cast under smooth force application. Via the forward stop position (in relation to the D (stop) position) you match the size of arc. Smaller arc is perfect for a tight overall loop shape. But it means less possible max line speed = less possible max distance. Wider arc means higher possible max line speed. Make sure to position the rotation close to the end of the stroke when using a wider arc (for shaping a tight loop). Using a longer stroke matches best for using a wider arc here. In order to be as efficient as possible make sure to move the rod hand along a straight line during the stroke. This straight line path may be horizontal (or even slightly upwards) for long roll casts or tilted slightly downwards for shorter roll casts. If you want a wide loop shaped roll cast just move your rod hand along a convex path and use a relatively wide arc while positioning the rotation during the whole stroke.
One more important point to focus on: Always make sure not to waste any of your forward stroke length and/or available size of arc. That means start the smooth acceleration right from the beginning of the D (stop) position. Many casters start very slow and then suddenly increase acceleration at some point along their stroke. Something I have seen in many slo-mos! This is because it is not that easy to start smooth acceleration right away from a static position, in which you have been pausing for quite a while! If you start your smooth (forward) acceleration slower than you would have to (in order to still be smooth enough) you will not reach the highest possible max line speed for sure.
Mostly the roll cast will be done in these two steps. If somehow you end up with a slack lined anchor, you may want to add a third step: 1a). Lift your rod hand slightly before making the forward cast (2.). This helps to reduce slack line again.
Hopefully this will help you to significally improve your roll cast.
To those of you having trouble with the leader to fully unroll: Make sure to position your (main) rotation near the end of your forward stroke/cast and stop the rod high (at around 10)! That simple? Yes, it's as simple as that. In my lessons it takes me no more than 2 minutes (the most) to get students to the point to be able to get the leader unrolled. This works on grass, on water and wherever you may want to roll the line out.
Rolly week to all of you!
All my best
Some pictures of the past (teachy) week: