There are two kinds of methods: destructive and non-destructive.

Destructive methods doesn’t allow to re-adjust after conversion. The only option is to undo the conversion and start again.

Non-destructive methods use layers so that the changes are stored in layers and original photos are unchanged

1. Greyscale.

(destructie)

Converting to greyscale discards all the color information in the photo.

Image -> Mode -> Grayscale

2. Image adjustments to black and white

Image -> Adjustments -> Black & White

3. Lab Color

(destructive)

1. Image -> Mode -> Lab Color
2. Channels window -> Lightness channel
3. Image -> Mode -> Grayscale
4. Ok

4. Hue/Saturation Tool – working on layer

The result is the same as method 1.

1. Layer -> New Adjustment Layer -> Hue/Saturation
2. Ok
3. Saturation = -100
4. Ok