What are the keys to creating a fictional language?

I am writing a fantasy story and in it, there are words and phrases that my characters use that are not in the ';common'; language. For these, I want to have consistency and want to make sure that there is some ';realism'; for the language I am creating.





The best I can think of is my limited knowledge of Spanish and how the words are formed, how they mean, etc. I am using this as my basis of verb format. (I hope that makes some sort of sense?)





Anywho, what are the best ways/keys to creating your language for fiction?What are the keys to creating a fictional language?
I would avoid an entire language if possible.


If you are shooting for slang I would really focus on how words are formed, search for origin in them.


Consider real world examples on slang then try to draw a conclusion about how those words came into existence or why they are used to represent that idea.


Then apply that same model to the words you are creating.