Well, first of all it's important to gather reference on the time and place you're trying to portray. what kind of clothing they wore, the climate of the place is very relevant, if you're using men's or women's clothing as the main theme of the outfit. You want to pin down what elements of clothing are very characteristic of that time and place.

After that's underway, it IS an assassin's creed character, so the hood is important, the red band on the waist is important, the gautlet with the hidden blade is important and other types of weapons as well. Again try to get to know what weapons were comonly used at the time, as far as guns and such go. Also, maybe she has a signature weapon? Depends on her backgound really.

At this point you should have an idea of how the outfit is going to come out, of at least a general guideline. You can go and make a more feminine look, but keep in mind these are people who run around all day, climbing stuff and fighting, so practicality and confort are a pretty important thing to pay atention to.
When an outfit is a bit complicated, designwise, I like to, as I did in the piece you saw, break it down to layes and components...

That's it I think HAHA a bit of a general idea, but I hope it helped?
Pinterest is a great place to go digging for reference btw!