There are a number of ways to do this. For instance, Lindsey Davis writes a series of mysteries set in ancient Rome and her characters speak slangy contemporary English. The MC has a smart alecky voice and from the start, you know that's what you're getting. In contrast, in Elantris, Brandon Sanderson makes up at least one slang word (colo? Is that right? I don't have the book in front of me) whose meaning is clear by context. If you're making stuff up, I wouldn't do too much of it because it's not going to have the same impact on the reader anyway and can make them work too hard.