I am creating this tutorial with the assumption that you have some basic knowledge about GIMP and can navigate your way around to some degree. If you need anything clarified, definitely leave a comment and I'll try to help as best as I can.
another thing to note is that I am using a modified GIMP. therefore the window is different from the stock version and things may be in different places than you are used to. Hopefully this won't be a problem.
The two images I used can be found here:
Step 1: Adding images/isolation
Also note that a feather brush is very useful in a lot of cases when you need to isolate something because the soft edge blends better with the rest of the scene.