What’s the proper way of using the 草 slang?
I mean, I can’t just stick it at the end of every Japanese word, right? On some combinations it will sound unnatural doesn’t it?

面白かったwwww sounds natural?
ラーメンwwww sounds weird right?

質問を翻訳

ラーメンwww doesn’t sound weird either.

For example,
Your friend sent you a picture of her eating ramen early morning.
You reply with the text ラーメンwww。（hahaha, you are eating ramen now?? ）

Basically there is no particular rule how to use it. However, you should be very careful with whom you are using it. Some people might feel that you are making fun of him/her, and get offended.