The usual way is to use a "Luminosity ask" (in other words, to select pixels in relation to their luminosity).
- duplicate the layer
- Color>Desaturate (in 2.10, I used the "Luma", in 2.8, use Luminosity)
- Open the Channels list, right click on any of the R,GB channels and "Channel to selection"
Now your light pixels are very selected and your dark pixels are almost not selected. You can then use Levels/Curve to darken the lighter areas and the picture will remain natural. Don't overdo it though because you haven't got that many light tones available in the initial picture:
- duplicate the layer
- Color>Desaturate (in 2.10, I used the "Luma", in 2.8, use Luminosity)
- Open the Channels list, right click on any of the R,GB channels and "Channel to selection"
Now your light pixels are very selected and your dark pixels are almost not selected. You can then use Levels/Curve to darken the lighter areas and the picture will remain natural. Don't overdo it though because you haven't got that many light tones available in the initial picture:
You can also futher edit the BW image before making it a mask for instance by darkening even more the dark parts. There also scripts around to create the luminosity masks automatically.
A good intro to luminosity masks: https://patdavid.net/2013/11/getting-aro...masks.html