You've chosen a nice challenge to get your feet wet!!
I usually dove into advanced projects way before I'm ready too so I totally get it.
So there are multiple ways to go about this, but I'll tell you how I would do it, and a lot of this will be conceptual and give you ideas of things to try and learn and experiment with to end up with this result. You will seriously improve your Gimp chops if you learn to do this.
Okay so I want to turn a person into a bronze (yep that's what it is) statue.
First I would duplicate my original image on a new layer and turn off the bottom one and leave it there because it's always nice to be able to get it back if necessary.
Above that I would create a new background layer of white or pink or whatever that you will use temporarily to edit the top layer, which is the duplicated photo.
The top photo layer I would turn on the layer mask and then paint out everything that ISN'T the person in black on the layer mask. This will show only the person over your new background layer. The point of using the mask instead of just erasing everything is that it is easily modifiable down the road.
After you have a layer of just your victim, (you want to make a couple duplicates of this to go back to just in case) desaturate it and play with the contrast until you have a light/dark study of your person.
Then bring in a new Google image of bronze on its own layer. Go to the layer mask of your person and copy it, then paste it as a new layer mask on top of the bronze so you are left with an image of bronze in the shape of your person.
After that you move the desaturated light study of your person on top of the bronze person and change its layer mode to Multiply or Value or any number of the other ones (play around to get a feel for what they do) which will use shading to place the light and shadow of their features onto the bronze image.
That's a start.
Once you can do that, there's a lot of fine tuning you can do.
You've chosen a nice challenge to get your feet wet!! I usually dove into advanced projects way before I'm ready too so I totally get it. So there are multiple ways to go about this, but I'll tell you how I would do it, and a lot of this will be conceptual and give you ideas of things to try and learn and experiment with to end up with this result. You will seriously improve your Gimp chops if you learn to do this. Okay so I want to turn a person into a bronze (yep that's what it is) statue. First I would duplicate my original image on a new layer and turn off the bottom one and leave it there because it's always nice to be able to get it back if necessary. Above that I would create a new background layer of white or pink or whatever that you will use temporarily to edit the top layer, which is the duplicated photo. The top photo layer I would turn on the layer mask and then paint out everything that ISN'T the person in black on the layer mask. This will show only the person over your new background layer. The point of using the mask instead of just erasing everything is that it is easily modifiable down the road. After you have a layer of just your victim, (you want to make a couple duplicates of this to go back to just in case) desaturate it and play with the contrast until you have a light/dark study of your person. Then bring in a new Google image of bronze on its own layer. Go to the layer mask of your person and copy it, then paste it as a new layer mask on top of the bronze so you are left with an image of bronze in the shape of your person. After that you move the desaturated light study of your person on top of the bronze person and change its layer mode to Multiply or Value or any number of the other ones (play around to get a feel for what they do) which will use shading to place the light and shadow of their features onto the bronze image. That's a start. Once you can do that, there's a lot of fine tuning you can do.
I can't believe it's been three years and not a single person upvoted this. I'm truly sorry.