It’s really simple, you can check the PSD in the zip file to see the construction (should be CS2+ compatible, at least).
The outline’s drawn with a 2px, hard black brush with brush size on pen pressure. Takes a few tries to get the right shinky line That layer’s then duplicated and collapsed down to make the outline bolder.
After that it’s pretty much primary school level of colouring within the lines using a larger hard brush. One extra layer for shading, one for highlights (both solid colours) - to match the 3-layer cell shader I wrote for my game.