This guy on therpf.com did an amazing job with the reboot version of Amazing Spidey: http://www.therpf.com/f24/mcl34ns-wi...ed-6-a-140975/
(I know you said Spidergirl, but figure you could use the same methods)
He went to a lot of detail and expense I'm not sure you're willing to follow, though. He got his fabric printed by a shop after he perfected his pattern.
However, I believe he also used some puff paint for the black lines. Maybe it was just on the prototype, I'm not sure. But it seems to have worked for a few others. Try searching therpf.com for some other Spidey posts, because it seems there are a lot more comic book character cosplayers there than there are here.