Here's how one blogger did it! Her fireplace turned out AMAZING: http://www.coastalcollectiveco.com/blog/how-to-whitewash-stone-diy-fireplace-makeover
Just use thinned down white paint. If latex thin with water. If use oil based thin with paint thinner. Just paint on with a brush in the consistency you like.
Here is one video, some just paint it on and others who want to see some brick through the paint rub off the excess with a rag... https://www.youtube.com/watch?v=XkVBm3bitEE
Here's a post that might help.
Watered down paint, bucket and begin brushing. I did our monster wall a couple years ago was EASY. Posted it here on hometalk
"Massive brick fireplace wall white success"