I'm pretty sure my oc is engraved into their head, is this a form of torture, or are they okay? I feel bad it's like keeping a pet artist in a cage and forcing them to draw for you .
First, if your art is on a white background, you'll need to select the artwork on a selection layer using the Bucket tool. After that, duplicate the artwork from the white layer twice. Change the top duplicate to a different colour, such as grey or green, then use Stroke (Outer) to make it expand. After that, use Stroke (Outer) again, this time with white or a darker colour.
Next, use the Bucket tool to delete the grey/green part until only the outline remains. Use the tool shown on slide 9 to move parts around if needed. Then, on a layer underneath the outline, use the Bucket tool to fill the inside with white. Once you're done, colour your line art.
This probably isn't the best way to do it, and there are likely faster shortcuts, but this is what worked for me. <3