1)Duplicate image layer
2)Turn top layer Black and white (desaturate is the easy way, but there are many other ways)
3)Paint black in a layer mask where you want the colours to show through.
I'm sure there are many other ways to get to the same place.
Those instructions would work for any image editor, Photoshop, Gimp or which ever. But the devil is in the details. for example there are many ways to make a mask, you can use the selection tool and there are many selection techniques or you could free hand draw the mask as suggested above. And then do you want sharp or fuzzy edges on the mask? masks don't have to be just black and "clear" but can have grey. I would suggest some "feathering" of the mask edges.
Next as hinted there are many, many ways to do a B&W conversion and it's kind of like if you had B&W film. You would choose a filter (red, yellow, orange, blue,..) and then you choose a film and then a developer and then how long to leave the film in the soup. All of these are artistic decisions. And then most B%W film photographers would tone their prints with any of several toners to make the blacks and whites either warmer or cooler. Finally what color shows through could be "tweaked" too. Maybe you want to fade the colors a bit.
What I'm getting at is that you should not just follow simple instructions and expect great results. There are 1,000 little details that effect the "look".
And then there is the problem of how to get the end result to the printer and have it exactly match the screen. Gimp is not color managed so some experimentation is required.
