How do I make snowy ground textures?

Hi. I'd like to make the runway/taxi/aprons look snowy. I have long experience placing library objects but Ive never done anything custom. (I have basic ADE experience.)

This is what I have so far...https://drive.google.com/file/d/11vU_gMwRA9YRxDa6gn6pR8jLKwEJZRqT/view?usp=sharing