How do you turn a raster image into vector paths in Sketch?
Illustrator is recommended but if you still want to do it in Sketch here is the way:
Suppose this is your image
I want to trace the top he is wearing. I'll simply draw a rectangle
Double click on it to see the nodes. You'll also see the inspector on the right has changed. You can make it curve or straight lines as you like. To add a new node just click on the side from where you want to start a new node.
Here is how it looks like. Not a perfect one but you get the idea.