Basically, Gen II sprites use "dithering", which is a concept similar to shading properly, except in places where you'd use a lighter color, you criss-cross white + the color. For darker shading, you cross black + the color. In the end it gives the illusion of shading and even of having more than two colors. I personally find it a super tricky concept to work with and am thus terrible at Gen II sprites entirely. You can find numerous examples of what this looks like all over the game though. Look at any Pokémon in it, really :P
You could maybe try practicing it by taking a Gen III sprite and redoing the shading from the bottom up, using just two colors, dithering where the more contrasting shades are. I doubt people will be too judgmental if it's "not perfect" and of course practice makes perfect.