Are you making up words? Chances are if you're writing fantasy or science fiction, you're making up far more than one. But almost any book can involve new language. Teen slang is an ever-changing miasma of made-up words. If your book has teenagers, you should probably have some kind of slang. Otherwise, they aren't really talking like teenagers.
But how make false words are too many? If I write out "The Glorxian sacrophods invexed the Halati metalikuds" you have some idea that <proper noun> <things> verbed some other <proper noun> <things>. But even if you have a degree in linguistics and figure out that the author was trying to suggest with the cross-linguistic "metalikud" that the Halati have a group that is above consolodation that doesn't really help you understand anything at all. But that's not because the words are made up, it's because you have no context for them. This is the first time you've seen these words, and they don't come with any context whatsoever, so you have no idea what they mean. If you provide the proper context, and make the language rich enough, (cough cough Elvish) everyone will want to learn that shizz. You know they made a whole language for the Avatar people? Cool, but no one actually cared about the language, because the world of that movie wasn't really compelling enough to make me want to pretend to be one.
Ooh, but what if someone's trying to take my unobtanium? Godzilla. If all your made up words are like unobtanium, then please, take all the made up words out. But for the rest of you, who have imaginations and intelligence, don't throw your made up words away. Sometimes they are beautiful, sometimes they enter the real language. I don't believe for a second that Shakespeare made up all the words English profs say "he" did -- I think that about 99% of them were common street language, which was never written down. And if you include a slang word that you and your friends use but nobody else, and it catches on? Think about grok, or frak. I at least hear those words pretty much daily. Frak is fun to say, and we all know what it means because it shows up exactly where fuck is supposed to be. But frak sounds funnier and doesn't come with any connotations other than BSG.
The key to getting your fwords into people's brains is to not make your book a language textbook, with long descriptions and definitions, but to leave those words like little crumbs of cake, a tantalising tidbit that you nab quickly and easily. Set a new word up with proper context, put in the proper grammatical slot and bam your readers will slurp up that word. Hell, if you make the language beautiful or compelling enough, people will learn the whole damn thing! Klingon appeals to those of us who love to yell consonants, Elvish to those who like to whistle and sing. My advice to making up words if you don't have any sort of academic linguistic knowledge is to just go with what sounds natural, and give us enough context to understand what it means. Don't give me a quiz on whether a lanitar has a long shaft or short shaft or whether the mechanism to create the spinning blade motion uses chains or gears or both. Just let me know that the mechanism whirrs as it slices through a thousand orks. (or orcs, whichever you want!!! Start a controversy over the spelling of your made up thing!!)
If you put in fwords sparsely towards the start, keep their usage consistent and supported, by the end of your book we'll be able to read "The Glorxian sacrophods invexed the Halati metalikuds" and go "OH SHIT!" instead of "Uh, what?" And that's what you're really going for.
2 days in, 28 more to go! Get those words, get those novels written!