 Re: Green and white ink on black sweat shirt
Without knowing any details about your equipment, my feeling is the white should extend to the entire picture, even underneath the green. You just lay down the green on top of the white so that the characters (and everything that you want to be white) is left "empty". In other words (aftet the white ink) there's no ink on the parts you want to stay white.
To lay down green ink alone on a black shirt might not really "bring out" the color since it's probably not enough to cover the black fabric by itself. That's why there should be an "underbase" white under the green ink. And if you really wanna do it HiFi, you might make a second white screen for a "highlite white" where you lay down just the white parts on top of everything.
But I'm sensing you're gonna go the DIY way so just lay down the white strong enough so it really looks white and seems to be covering the fabric underneath. After that all you need is the green ink and you're all set.
There might be a lot more I should tell you but since I have no idea what method and inks you're gonna use then the rest is left for trial and error. | 
