How much of this language are you going to use? Single words? Phrases? Sentences? Paragraphs?
If it's just a few words or phrases, make up a few, be consistent in their usage, and call it a day.
If you're carrying on entire dialogues in this tongue, I would first recommend "Don't overdo it." For the purposes of your question (and I Am Not A Linguist, so take this cum grano salis):
Write your dialogue in English (or whatever the language of the book is) so you know what words you need.
- Create some stems. These are the core bits which you build the word
from. Taking Romance languages as an example, amo(r)- for love,
mang- for eat, blanc- for white, petr- for stone.
- Create some prefixes and suffixes.
- Combine those to create words.
- How does the sound of the language reflect the speakers? (Papa
Tolkien was the master of this. The Elves' speech was liquid and
trilling, with Ls and Ss, because they were beautiful and serene, but
the Dwarves spoke in harsh guttural sounds because they were a harsh
- Are there diminutives? ("Little" something) A word or phrase you tack
on to a name to indicate formality, informality, family, familiarity,
If you want to be thorough, you will have to think about some grammar.
- Does the verb come right after the subject (Romance languages) or
at the end (Russian)?
- Does the verb conjugate (so word order is important) or decline
(words can be in any order)?
- Does the verb ending change with time (present tense, past, future),
with number, with gender (Hebrew)?
- Think about word forms (the noun vs. the adjective form of the same
word: book vs. bookish)
- Articles (a, an, the)
- Contractions (dropped or elided letters). Don't throw in apostrophes
to be decorative; they should mean something. In English the
apostrophe means letters were dropped. (the possessive 's
is attributed to the archaic phrasing "John his book → John's
- Do characters from different classes or different geographical
locations have different variants on the same essential word?
An example in practice:
In Mercedes Lackey's Valedmar universe, there's the root language of Kaleda'in, which has two major branches, Tayledras and Shin'a'in. The word for "love" or "beloved" is ke'chara in Kaleda'in, ke'a'char in Shin'a'in, and ashke in Tayledras. The word for "gay" translates as "one whose lover is like the self," and the phrase in Tayledras is shay'a'chern. A gay person refers to his/her spouse as shay'kreth'ashke. In Shin'a'in, the word for gay is she'chorne. You can see the cha(r)- stem moving around in that sequence.
ETA more examples from the same writer:
Tayledras, in its own language, means "Hawkbrothers."
In Shin'a'in, the same word is "Tale'edras," inverted to be "Brothers of Hawks."
Another Shin'a'in clan is called "Tale'sedrin," which is "Children of the Hawk."
Shin'a'in terms for their horses are "dester'edre" (wind-born siblings) and "jel'sutho'edrin" (ever-younger clanschildren).
I seem to recall "jel'enedre" means "younger sibling."
A Shin'a'in term for two people who have sworn blood-sisterhood is "she'enedre." (and there's the she- stem for "same" from shay'kreth'ashke of Tayledras)
Shin'a'in itself means "people of the Plains."
The plains in question are called "Dorisha Shin'a," the "Plains of Sacrifice."