I understand these are used for practicing roll casts, when practicing on the lawn.
I’ll just be using a 5 or 6 wt. rod.
Thanks,
Mcmanus
Jason Borger has a great description in his book “The Nature of Fly Casting”.
Essentially, you have a monofiliment leader tied with knots every foot or so. On each of these knots leave about 3" of tag ends. That is, leave about 3" of extra line exposed at each knot.
As those extra ends drag through the grass they simulate the drag you get on the water when doing a roll cast.
I’ve never made one, that’s just how I saw it described. I practice my roll cast on my pool. I lay out some line on the water and see if I can roll cast across it and make it land on my dog (no hook). I think it’s a game. He thinks I’m lucky that I’m the one that feeds him.
Cut some tippet into 10" or so sections and tie them together with blood knots.
Leave the ends of the knots long so you have somthing to drag on the grass. Like the previous poster says 3" or so.
Make it about 9 or so feet long and cut it down from there to your liking.
use an old spent leader. tie a piece of yarn on the end
Thanks, guys
I have used them, many tag ends dragging in the grass and the yarn ‘fly’ also assisting. It does work, not as well as actually being on the water, mainly because it is the 20 to 30 feet or so of fly line in the water, as well as the leader, that you are using to load your rod. On the grass you only have the knotted leader. But it gives you enough drag to get the feel of doing a roll cast. You will find that once you are actually on the water the cast will look a lot better and go a lot farther.
I have often wondered about using an old, old fly line and tying in a bunch of mono leader knots with tag ends, along the first 20 feet of the line, about every foot or so. That should add even more drag for a better load, but I haven’t tried it.
Larry 
—sagefisher—
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.