To convert a color photograph to black-and-white, go to the Develop module, and do the following.
There are four "stops" on the way to a black-and-white photograph.
Let's say you already have a a black-and-white photograph.
And, you didn't convert it to black-and-white from color.
The photograph's file may not have color information.
If so, you won't be able to modify the tones by modifying the colors, as described below.
1) In the Basic panel, select Black & White.
This setting desaturates the colors.
In most cases, the photograph will need more work.
2) Go to the HSL/Color/B&W panel, and click the B&W section.
3) Click Auto at the bottom.
You can set Lightroom to apply the Auto adjustment whenever you click Black & White in the Basic Panel.
Go to Edit > Preferences (Windows) or Lightroom > Preferences (Mac).
Click the Presets tab, and select Apply auto mix when first converting to black and white.
4) Return to the Basic panel.
5) Use the tools to further adjust the tonal values of the photograph.
6) Return to the B&W section in the HSL/Color/B&W panel.
7) Adjust the tones of individual colors.
While the photograph you see is black-and-white, the photograph's file retains the color information.
Let's say you're editing a landscape—that had a blue sky—before you clicked Black & White.
Do the following.
a) Click the target-part-of-a-photograph icon in the upper-left corner of the sub-section.
b) On the formerly-blue sky, click, hold, and drag up-and-down.
The tone of the formerly-blue sky will be modified.
c) Click the Done button when you're finished.
The button is located below the photograph on the right end.
There are many presets for black-and-white photographs.
Go to the Presets panel on the left side of your screen in the develop module.