Once you get the first side done, copy your line art and flip it. Once you do this, line up the sides so they are almost flush. At that point, you’ll discover that certain spots look flat or odd. This is your chance to go in and clean that up and also blend the middle a little bit so it will feel more unique.
Adding colour is quite fun. Create a new Layer under your line art. Lock the layer that has your line art on it, and also hide or click off of the layer with photo. You want to avoid any distractions and work with a clean background. First you want to drop a base colour down.
I generally work in greyscale, then change it to colour later to save me some time, however it’s your call. You don’t have to be perfect when dropping in colour, it’s a matter of just filling in the areas that you want to pop out. Take time to throw a few shades of darker colour or lighter colour for shadows or light reflections. Doesn’t have to be over the top, but enough to give it some depth.