Need help, Masters Copy - WIP

[EmMe]

Hi all, this is actually my first work I've posted on the forums, I'm currently in a community college art class, nothing too big or special, but our final is a copy of a masters drawing, and I chose (from a book) "a moorish archer" by Piazetta, I just started it a bit ago and I'm really confused on some things, it's my first time using pastels, I'm using brown paper with black and white soft pastel.

My problem is, I dont understand how I use the black and white pastels together, when I put on the white pastel over some of the black it just ends up blending and making a bad gray color, am I supposed to do it all in black first, then spray it and go over with the white so it won't blend? Right now I'm at the point where I feel like starting over cause I messed up on a spot (might be visible in the picture) so yeah, excuse my stupidity, and thanks.

P.S. the silver outline is from a pencil I used showing up from the flash.

[Nistarn]

Dont apply them on layers, save the space for blacks, and for whites, or in other words, think of values, and there is no acentuated black next to a highlight. For halftones, you can do crosshatching with black and white in the right proportions. i hope i helped in any way.
use this to train an do another one. : )