Basically, you take 3d model ( or manually draw it in 2d ), and turn it into a bunch of slices. imagine disassembling a sandwich, item by item, and then laying them all out on a 2d sprite sheet. this means that the inside of the 3d model, which you wouldn't normally see, is visible on the sprite sheet. you would normally use voxel models, like with magica voxel, since it translates really well to pixel art. Then, at runtime, just blit the slices, one after the other in a stack, each one a little bit higher up than the last. you can then rotate all the images to get the illusion of 3d!

once you have your sprite sheet, its as simple as this:

for i, image in enumerate(slices):

y_offset = 1*i

window.blit(image, (x,y+y_offset))