What steps do I take to painting a brick wall mural?
I would avoid using chalk or charcoal for laying out the grid and/or sketch lines as it could cause finish-coat adhesion problems. Assuming this is a large scale project I would use a carpenter's pencil (hardware store) to lightly mark the grid and to sketch your design; no need to rub off as the paint will cover over the pencil marks. 

Don't draw solid grid lines, use a straight piece of lumber and draw intermittent marks for the grid. 

As for paint, it depends. Pre-tinted exterior house paint might be a good idea if large areas of the design are a single color (they can be duplicated with fair accuracy). If the design is intricate with many hues required then it would probably be more economical to purchase smaller containers of acrylic paint (art/hobby supply store) and mix them yourself (colors will not be easily duplicated though).

Large area brushes: hardware store, and do not skimp on the quality; good brushes= good coverage and finish.

Intricate design brushes: art/hobby supply store.