1. Open file
2. Control J makes a new layer
3. Control + Alt+ U makes the new layer black and white.
4. click b for brush.
5. Hit or hold "[" to make the circle of the brush smaller or "]" to make it larger
6. now on your pallet where it has the white and black box, on the left, basically that lets you switch back and forth between layers, you click the white, because you're basically ERASING your black and white to see color. Best part is... you can click "X" and it swaps back and forth from either erasing your black and white, or putting it back on. Makes it pretty easy, and if you go over, kinda like coloring as a kid, just hit X and reapply the black and white over the mistake.
