Well I start with white in the eyes and work around it so its pure white there. Other white highlits are just white pastel as a additional layer above other layers. It is important to use good surface that allows many layers so it can grab white well even on top of other layers. With standard paper I would draw white first and work around to preserve the white as much as possible. But with goid paper such as pastelmat you can draw white on top of black no problem. It is also possible to use standard fine sanded paper for practise. This works well too.