Incoming wall of text.
Download Gimp if you don't already have a better editing tool. Gimp is free and has everything you need to do these kinds of edits. Play around with it for a while, get a feel for it. It isn't super intuitive but there are ass loads of tutorials out there for doing things specifically in Gimp, anything you could imagine.
1. use layers, lots of layers. Use one for every different color, your shadows and your lines. This will help keep things tidy and help you fix shit when you fuck up. also never fuck with your original image, leave it as is. Everything you do just goes on top. They should be arranged as lines on top, then your various colors, then the base image.
2. Enlarge the image before you start. I usually try for at least 5,000 pixels on the smallest side, though sometimes that isn't possible. This helps you to match line size among many other things. when you shrink it back down to original size to post it will help blend your edit with the rest of the image.
3. Draw a base skeleton for the image. Draw out where the rib cage starts and ends, draw out the center line. Draw out which way the shoulders are aligned. and then draw out the edits you want to make, preferably in a large, vibrant color. This is your guide. use thick, obnoxious lines. It doesn't have to be pretty, it just has to be a guide This is how you make sure things are going to look the way you want them to look before you actually start putting down lines and colors. I usually set the skeletal base layer at about 30% transparency and let it sit on top of all my other layers.
4. The color match tool is your friend. Just because a line looks black doesn't mean it is black. Pick all the colors and line colors from your base image.