How to say...

"Certa tanquam puellam"

http://translate.google.com/translate_t?hl=en&ie=UTF8#

If I may make a couple of suggestions:
I'd use sicut rather than tanquam.
I think you want puella (nom.), rather than puellam (acc.).

I just used online translation, actually one site to obtain phrase and two others to confirm. The one to use is the one that looks best on a tat - but, I guess you don't want a translation that goofy (even though it is a dead language).

Tanquam looks pretty neat; but sicut absolutely rocks and I also think that puella sounds neater. So, I would go with your changes for sure!

"Certa Sicut Puella" on a heart with a dagger in it dripping blood.....

I used to have a tattoo like that.
But it peeled off.