Not sure what kind of pastels you are using or brush, I recommend soft pastels and a brush made with hair bristles and not synthetic, but there are multiple ways to streak with pastels. You can use a Q tip to put some pastel on the model and then feather it out with a brush. You can do a complete pastel wash then leave some streaking as you are cleaning it off, it is the easiest way. You can also apply some pastels to the leading edge of where you want streaks then use a wet paint brush to streak it. Just be aware that your clear coat over your pastels must be applied at a very low pressure or you will lose a lot of the pastel you apply. I recommend using some type of wet technique and once it is dry it doesn't spray off as easily when applying the clear coat. Good luck with it and check youtube too.