The main problem is having to make the sprites and only having a tiny bit of room to do so in. this is to make it fit the box in the sprite editor.
I recently added Zorua, Zoroark, and a Zoroark evolution.
Then I added an Electric-Flying Pokemon that has at least two evolutions.
The backsprites were especially tiring to work on, then I had to work them into the list of pokemon in the game.
Putting it into the sprite editor also doesn't always work, so you have to change your picture a few time. I don't like how my final Zorua evolution worked out to look like, but the basic idea was there. Fortunately, I was able to use a re-paletted Arcanine backsprite.
here is some of my work.... I don't know if there is a type of SpriteEditing help thread. That would be a good idea though.