How do I do shading like this?

I know it might not be very helpful, but the elephant in the room here is that this would be better done without even switching on your computer. The whole beauty of that tattoo flash style is that it's hand-drawn.

Sorry, John, I was interrupted by sleep.

I think you would be well served by the gradient mesh techniques in the video that CHMprepress posted. If you go in that direction, don't try to fill your tracing with color—use that as your top layer, overlying gradient mesh and other artwork beneath. Work in layers, adding them as needed. Turn off your tracing layer (your pen tool work) as you work on colors, just turn on to check your progress. Don't try to get all the details in at once—eyes, mouth, dimples, strong shadows, and other finesse work can be added on above a base layer that just has the facial skin tones and major shading. Remember that you can use transparency anywhere in a gradient mesh, which is useful for blending with layers beneath.

Save often, and save multiple versions as you go, in case you find yourself in the wrong direction, and don't want to do 500 undos.

Starting a gradient mesh from a rectangle, regardless of the shape you want to end up with, will give you maximum control and predictability over mesh lines that you add as you go.

You'll notice that this assumes you'll use Illustrator. It's the one I know best, and the one I'll choose whenever possible.

John, Peter,

Actually, TIL is at least in British context reckoned as an obsolete form taken up again; with a bit of luck it may rise to prominence and be seen as just the other standard form (alongside its other meanings such as sesame).

Jacob, it's nonstandard, but it's not wrong.

Ah brilliant thank you, I haven't used the mesh tool before so will need to find tutorials on that first i think but yes this looks like what i need! Thank you!