Here's a situation almost all fanfiction authors encounter at one point or another. You're right in the middle of your glorious epic novel about Thingol's life story, and then, without warning, an original character shows up. A messenger brings dire news to the Queen. And he needs a name. Unfortunately, because this is the Tolkien fandom, you can't just call him Byron or Jeff and be done with it. Nor can you make up some random fantasy name like Qhaer'Yaah off the top of your head.
(Well, okay, technically you can, but you'll be wrong and people will bitch about it.)
Coming up with a reasonable name for that original character may seem like a daunting task at first, but it doesn't need to be. Here are a few tips that can help out with the process.
1) Keep it simple.
Take a look at some of the names Tolkien created. Trust me, they just leap off the page with originality. Haldir = tall man. Arwen = noble maiden. Glorfindel = golden hair. Celeborn = silver tree. Thingol = grey cloak. Aredhel = noble Elf. All of these names follow the very simple pattern of adjective-noun, and not one of them is particularly outlandish in its description. Instead of trying to translate a specific phrase like 'graceful princess of the dancing light' into Sindarin, stick to very brief, generic meanings: green jewel, faithful hand, fair song, glorious star.
2) Don't cop out and call everyone Noun-ion and Adjective-wen.
Keeping that 'simple' rule in mind, it's also important not to oversimplify. A few characters with names ending in -ion, -iel, and -wen are fine, but when you have a series of OCs named Laerion, Morion, Brithion, Gilion, and Nenion, it gets a little silly. Try to set the limit at one character with each common suffix, and come up with something a little different for everyone else. While Tolkien did provide several -iels and -wens, male names ending in -ion are relatively uncommon.
3) Don't tailor the name too much to the character.
This may seem counter-intuitive, but creating a name that's too appropriate to your character can do more harm than good and tip you over into the realm of cliché. A warrior does not need to be named after swords or battle any more than a musician needs to be named after flutes or songs. Resist the temptation to give all of your OCs occupation-specific names. The generic meanings listed in point one above would be perfectly fine across the board. 'Green jewel' works just as well for the captain of the guard as it does for the King's minstrel.
4) Sound is more important than meaning.
It's nice to be able to give your character the meaning you want, but sometimes it doesn't work. If the name sounds weird, or is too long or hard to pronounce, don't use it, no matter how perfect the meaning is. Having a name that looks good on your computer screen and sounds good when you say it in your head is vastly more important than what it means. To give an example, say you were set on an OC with a name that means 'pure voice'. It's a great meaning and perfect for your character. Unfortunately, it translates into Sindarin as Puiglam. I don't think you need me to point out how terrible this name is. Don't stubbornly go ahead with it. Find something that sounds better. Call him 'green jewel' instead: Calemir is way nicer.
5) It is possible to cheat at Sindarin.
For many people, the worst part of making up original character names is worrying about whether or not they're linguistically correct. Especially when it comes to Sindarin, a language so confusing that it cannot be reasonably summarised in this paragraph. But, luckily, there are ways to cheat at Sindarin and make yourself a correct name even if you know nothing about its rules and pitfalls.
The easiest way out: pair a word ending in a consonant with a word beginning with that same consonant. The two matching consonants will overlap. Aran + nen = Aranen. Hûr + ras = Húras**. Lach + hiril = Lachiril. The only ones to watch out for is the letter S, which will double: falas + sîr = Falassir*. Letters L, M, and N can also double, but S is the only one that must.
Also very easy: pair a word ending in a vowel with a word beginning with F, L, N, R or TH. Adui + loth = Aduiloth. Minai + thôr = Minaithor*. Lû + roval = Lúroval**.
Other safe combinations: any word ending B, CH, D, DH, G, R, or TH + vowel. Ereb + ael = Erebael. Ardh + aran = Ardharan. Mîr + ethuil = Mírethuil**.
In all scenarios, avoid words containing the diphthong AU (as this frequently becomes O or Ó depending on its position within a compound) and second words ending in consonant clusters (MP, ND, etc., as these frequently simplify into a single letter at the ends of compounds).
*Circumflex accents, in final syllables, are always dropped.
** In non-final syllables, circumflex accents become acute, unless they stand before a cluster of two or more consonants. In that case, the accent would be dropped altogether.
6) Keep a list of names ready to use.
If you're on a roll of creating names that look and sound good, don't stop. Keep a list of generic names on hand so you're ready for the next throwaway OC. You don't want to be spending half an hour coming up with the perfect name whenever a guard appears or somebody's sister is mentioned. A good list will save you time when it comes to naming those minor players and filling in family trees.
7) Do not, under any circumstances, just make up some crap and hope nobody notices.
Because somebody will notice.
Linguistic Foolery:: Not Just the Son of That Guy: Creating Effective Names for Original Characters
© Darth Fingon
http://www.silmarillionwritersguild.org/reference/linguistic_foolery/not-just-the-son-of-that-guy.php