Creating B&W images

[godek]

I don't remember how or what it was. I learned this in a photography class in college and since have forgotten it. I think it was toning or maybe split toning or perhaps it was adjusting the contract but it had to do with using the eye dropper. I think the eye dropper was for finding the darkest(shadows) and lightest(highlights) areas and using that to get the perfect black and white for that particular image. I think this was done after the channel mixer with the contract tool then you used the eye dropper to get the measurements.

How do I do this in gimp? I am using gimp development version 2.9.8. Is there a tutorial somewhere?