A good starting point is to inspect the individual red, green, and blue channels. One channel often has better tonal separation or contrast for your subject than the others, and that helps you decide how to build the conversion.

Common methods include:

• Desaturate / set saturation to zero: simplest, but often gives a flat result.
• Use a single channel: sometimes one channel already looks best in grayscale.
• Channel mixing: combine red, green, and blue in different proportions to control brightness and contrast between colored areas. This is often the most flexible method.
• Optional tonal adjustments first: applying curve or histogram-style adjustments to channels before mixing can further improve contrast.

In Photoshop or GIMP, the Channel Mixer is especially useful because you can emphasize or reduce the influence of specific colors. For example, if two objects differ mainly by color, changing channel weights can make them separate more clearly in black and white.

So the practical workflow is: view each channel individually, decide which tones you want lighter or darker, then use channel mixing to get the best balance while preserving overall luminosity and avoiding clipping.