Had a go at making these, very easy - get a crayon and with your paper pressed against the board give it a rub - you get a perfect guide.
Then make your 'master copy' by tracing over the crayon lines, photocopy as many copies as you like from your 'master'.
I have only had a couple of goes and I must admit, it makes you more aware of where your darts are going.
Too early to say if it improves you as a player but I will certainly have a few more goes with it on different targets, add to the variety of practice routines.
Cheers,
Greg