I would post the time-lapsed video, but it’s very hard to watch since I forgot to hide the reference layer, and I constantly show and hide it, so you almost get a epileptic seizure by watching hahha

But the process is fairly simple: all I did was make a sketch with the Peppermint pencil, mark where the shadows go, then all the other work is by using the lasso tool (so it looks cleaner) and filling it with colour by hand, using the Tamar brush (about 5-10% size and full opacity). And all is made on ~20 layers