1. Open a photo for editing in Photoshop
2. Begin by creating a new adjustment layer by going to Layer > New Adjustment Layer > Levels
3. Select "OK" on the dialogue
4. The Levels adjustment works by adjusting how black, white, and grey colors in the image are displayed. The black slider on the top (input) histogram will correspond to the black slider on the bottom (output) bar
5. Dragging the black input slider closer to the center will cause more colors to be displayed as black
6. Dragging the black output slider closer to the center will cause more black colors to be displayed as grey
7. Similarly, dragging the white input slider closer to the center will cause more light colors to be displayed as white
8. Dragging the white output slider closer to the center will cause more light colors to be displayed as grey
9. Dragging the grey input slider affects which color will be the "middle" color in the image and adjusts both dark and light colors
10. Adjusting the RGB channel will affect all of the colors in the image since it adjusts black and white; use the dropdown to select the Red, Green, or Blue channel for a different effect
11. When using a color channel, dragging the black input slider to the middle will reduce the amount of that color, while dragging the black output slider to the middle will increase the amount of that color. Dragging the white input slider to the middle will increase the amount of that color, while dragging the white output slider to the middle will reduce the amount of the color
12. The black and white eyedroppers can also be used to select the colors that should de displayed as black and white in the image