ok, these are just suggestions, but here goes.
For the hair, I'd use a long white wig, shoulder length at least. Probably with some choppy layers, but not heavily styled as Scar has messy hair.
Maybe some good quality white face paint for the scar. You could make a stencil out of posterboard or just paint it on free hand.
Of course some bad a$$ sunglasses.
A short version of his yelloy jacket, maybe only about the length of your rib cage. It has a white cross design on the back that runs down the arms. You'll want that.
Jacket similar to these:
A white v-kneck tank top and a black school girl style pleated skirt. Knee high socks. Shoes of your choice.
Oh and the tattoo drawn on your arm, of course.
Couple of Scar reference pics.
And the out fit similar to Ed's in this skit
Hope these suggestions help.Please post pics when you're done. I don't think anyone has done a female Scar before. Very Original.