I might have been a little vague in the steps.
I'll try to insert some pics here to show on a nice pic of Marley i just pulled from the web
that I just made ready to print on a shirt.
I started with my dpi @ 300
The first image is the file in grayscale.
The Second image is after making some adjustments using levels.
Then applied a bit of tweaking with curves to bring back a tad of gray. This makes a good halftone dot size variation.
The halftone settings i used to demonstate me printing on a 305 mesh screen. An easy way to figure your frequency would be to divide your mesh count you want to use by anywhere from 3.5 to 5.
The final image is after using the magic wand tool to remove the white from the background.
Send to the printer using your RIP and you should be good to go.
If you image is looking like a negative, could try to inverse your art?
Hope it helps!