Turning the skull layer’s opacity down allows you to see where the two layers overlap. Now use the Polygonal Lasso tool to select the areas of the skull that need to be removed – you want the flames that are on top of the skull (such as the ones coming out of his mouth) to appear solid. Delete the linework on the skull layer where it overlaps the flames.
Once you’ve removed the necessary linework, duplicate and reflect the flames, as you did in step 6. To complete the skull, go to Image > Adjustments > Threshold and use the slide bar to make the image solid black and bold. Using Threshold gives the linework a solid black fill. Now the skull linework is complete. As this design is intended to sit on a black T-shirt, we need to invert the illustration so the linework appears white and will show up on black – hit Cmd/Ctrl + I.
Now let’s add some bitmap shading. Using the Magic Wand tool, pick out areas of the skull you want to shade – perhaps begin with the top crown area and work your way downwards. Once you’ve selected an area, create a new layer and select the Gradient tool. You need to make sure the gradient is set to fade from white to 0 opacity (see the screenshot), then fill the area with gradient. Each gradient area should be on a separate layer.
You can experiment with this; I decided to shade the helmet on the skull’s head more solidly than the jaw and eye areas. This makes the crown feel more solid and distinguishes it from the face.