I'm still learning how to use GIMP's pen tool, myself, and I wanted to show what I've learned so far.
I had such a problem figuring out how to do curved lines, and I could not for my life find a tutorial that would help. So I made my own.
If I'm not much of a help, then trace your own artwork for practice. That helped me so much.
I wasn't expecting the Pepper line to look like that...