i did retyping a lot, like a lot, but my works are lost and i can tell you about retyping a pokemon to normal.
it is very difficult, since normal type pokemons came in every shape, every colour. but they have things like this in common
pink-pinkish colour(yellow comes second)
fur. lots of fur.
blank areas.(like consider a ursaring, it has a very very big and blank body right?)
being not too much spiky(like not having horns or something like that)
these things make a pokemon to look like they are normal type imho. they aren't %100 accurate homever, but i guess when retyping a pokemon, if you use these things carefully, pokemon will look like normal type.