I'm not familiar with GIMP, so you have to translate it yourself.
There are several ways.
I would simply use the "polygonal lasso tool" and do it manually. The polygonal lasso tool is a tricky one but in my eyes the only one that really works if you want to make things look good.
Make sure you have the starting / source point in the middle, when you do the innner ring and outside if you do the outer border. Create the outside ring with several starting points.
The severeal automatic selection tools work best with strong contrasts and you have to be careful with the "refine edge" settings. The results are often mediocre and make the model/object look like a cookie that had been nibbled on by mice (like on the main page of the h i g g i n s website
).
A second (5 minutes) way:
1. Make a second, empty layer. ("outer ring layer"). Set the transparency of the "outer ring layer" to 50%
2. Make a round selection on the "outer ring layer" with the elliptical marquee tool. It should fit the hoola hoop thing more or less. Feather = 0pt!!!! Fill it with a colour. Select "transform selection" (ctrl and t) so you can move and modify it till it fits the outer border/end of the hola hoop exactly. Use the different perspective tools to make it fitting.
3. Duplicate this layer then ("inner ring layer"), hide the "outer ring layer" and repeat it but this time with the inner border of the hola hoop.
4. Make a selection of the inner ring and then select the outer ring layer, make it visible again and delete the selection.
5. Hide the inner ring layer
6. You will have a black fake hola hoop ring on the outer ring layer that should fit the hola hoop ring. if not, adjust it again with the perspective tools. (if you have a pic of a good ring you can also skip 3, 4 and 5)
7. Erase the parts of the hola hoop that are hidden by the boy. Again: feather = 0 pt. Esp. at the should of the boy it will be a little time consuming.
8. Select the rest of the fake hola hoop
9. Go to the background and press Ctrl and J.
10. You should have a layer with the hola hoop and can change it to any colour etc. pp.
If the border is too hard, use gaussian blur on the fake ring or the softener to make it look better. Delete any parts of the shoulder and hand that are still visible.
Perhaps "contract" the selection or "defringe" the single ring (Layer - Matting - defringe)
Set the setting of the layer to "colour". I think it works better with light colours.
And there you are.
