How do i tell this girl i like her?

Im really starting to like this girl, but shes not really over her ex completly. and she is also my my ex's friend. We talk everyday almost all day at that. and we flirt and we even gave each other little nicknames. My ex doesnt know we are talking and its been a few months now. I really like her though and i wanna know how to make this happen and not keep it where it is and just get eff'd and end up in the friend zone