I used a PNG that was black
These are the setting that worked best for me
Engrave Power 30
Engrave Speed 500
Engrave Resolution 75
convert to dots and set to 50, focus at material
It gives a very nice screen effect, I also made a frame from Medium acrylic that I taped the Mylar too, This kept it stretched tight and also got it up off the bed a bit.
this also helps as it keeps it up off your material you are printing so you can get a good amount of ink in the screen before pressing it down to the fabric. I am planning on doing a video tutorial, but it will be a few weeks before I can get to it.