When authors put a created language in the mouth of one of their characters, they have to decide how to get the sound of the language in their mind to the mind of the reader. This is accomplished by the miracle of writing. We mostly take it for granted, but a second’s thought will bring to mind just how important and miraculous the technology of writing is: from my own private inner state, I can create a series of visual marks that will induce that same private inner state in someone else, even if we are separated by thousands of miles or thousands of years. For the author, a good writing system is no less important. Creating a language for characters to speak can allow the author to add a new, subtle dimension to the world they create. But readers aren’t going to get the full flavor of the language if it’s not written in a way that allows them to understand the way you want that language to sound.
Some authors create their own script for their fictional language (e.g., Quenya, Klingon), but many simply use the Roman alphabet. There are pros and cons to each side. On the one hand, creating an entirely new script gives the author artistic control over how the language looks, which can be an additional way to get across some feeling of the created world. On the other hand, without at the very least transcriptions into the Roman alphabet, the reader won’t know how to pronounce the created language. There is also the difficulty in typesetting and printing created writing systems; more than likely the only way to include them will be as images rather than text, especially for online publications. Because of this, it’s necessary, even in creating a new alphabet, to have a reliable way to render your language in regular Latin characters.
Many authors create a language based on a real human language they are familiar with, often English or German or one of the Romance languages (Spanish, Italian, French, etc.). Tolkien based Quenya primarily on Finnish. Working from a spoken language has the advantage of lending naturalness to the created language, and makes less work for the author since in such cases it can be advantageous and useful to borrow syllables or even whole words from the influencing language. This type of process would best fit a language being spoken by humans and human-like races, either on a past or future Earth, or a fantastical world based on Earth. On the other hand, it might be inappropriate for an alien language if the goal is to create a language that sounds unlike anything the reader has encountered before. Notice I said “sounds”, not “looks”; when creating a language it’s necessary to focus on the fact that ultimately languages are sequences of sounds created by a vocal tract, not collections of marks on a piece of paper. Writing is used to record language, but is not language per se.
One of the biggest mistakes authors sometimes make is to use a writing system or Latin transcription that is not phonetic. (Here I use the word “phonetic” because it is most widely in use to describe a writing system that records unambiguously how the language sounds; however, in formal linguistics such writing systems would generally be better termed “phonemic”.) A good writing system should indicate exactly how a word is to be pronounced, with no ambiguity and no guesswork. The average reader goes through about 200 words per minute; don’t make them slow down to figure out how to pronounce Xqbqf. Stringing together random letters can be a fun way to hint at the exotic, but it doesn’t work well in creating an organic, believable world.
Good writing systems go by the principle of mapping a single letter to a single sound. By this standard, you might notice that English has a terrible writing system (from the standpoint of pronunciation; it’s quite good for indicating etymological history, but that’s not relevant to our discussion). In English we have silent letters (“through”), single letters that create multiple sounds (“x”), and letters that sound different in different words, or even different places within a word (compare the different “t” sounds in tin vs. atom vs. addition). That’s not to say this principle must be obeyed at all costs. For English speakers, it would be much more natural to use “th” to indicate the sound at the beginning of the word “thin” than to use “g” for this sound just because your language doesn’t have a “g”. That’s not to say that you have to use English spelling conventions for a Latinate writing system. After all, English spelling is notoriously inconsistent. Just make sure to give your reader a heads up on what letter indicates what sound. As clunky as it can be, sometimes the best way is to offer up at the very least a pronunciation guide. There can be a lot more elegant if they’re worked into a short description of the language, either in the mouth of one of the characters or as a preface or appendix. Don’t underestimate people’s interest in knowing the technical details of your story’s language (whether you like it or not, there are speakers of Klingon).
As a linguist, I’d love an IPA-enriched pronunciation guide with every story I read that has an invented language, but I know that’s asking a bit much. However, even without linguistic training, it’s not difficult to provide the reader with a pronunciation guide that will enrich and ease their voyage into the language you’ve created. Even if you are familiar with the International Phonetic Alphabet, it’s still good to include layman’s descriptions for those who aren’t, e.g., a = a as in “father”, c = c as in “cat” (not as in “lice”), etc. You don’t have to pick a spelling system that preserves common English values, but please, be consistent! Don’t have an “e” represent the “a” sound in “pale” for one word and then the “i” sound in “machine” for the next one. This will rapidly leave your reader confused and possibly angry, depending on their passion for transparent spelling systems and linguistic training. The more difficult it is for the reader to decipher your writing system, the more they’ll be transported out of your story and back into their boring everyday life where everyone speaks English.
Clarion Blog 
A question on this topic: as an amateur linguist who is at least familiar with the IPA (in a self-taught way, that is; never formally trained) – what value would there be, really, in providing an IPA guide to an invented language in a published work? I can’t really say for certain, but my suspicion is that there are rather few readers who even know what the IPA is, much less what the characters in IPA actually sound like. The latter idea, of a guide that says “a = a as in father” seems a lot more useful to a general reading audience, wouldn’t it be?
I’m asking because I’ve wondered to what degree a publisher would want to invest page-count in a detailed IPA analysis of an invented language if they were still going to have to do the latter to get the gist across to most readers… and so I was wondering if a better place for this detail, for those that want it, wouldn’t be on the author’s site?