How to invent a new language

Today I want to tell you how I invent new languages for my fantasy series 'Floating'. I hope this will help you who write stories as well.

Step 1: Sound
Firstly you have to think about the surroundings of the language: Who uses it and how do they live? This can influence the diversity of words, the number of synonyms for a word and also how complicated the language is. I prefer to invent more simple languages, so they are more simillar to english grammar than to german grammar.
What does the language sound like? Is it a soft language with lots of vocals or is it a harsh language with lots of konsonants? Which letters are mostly used and which not at all? Do they sound different from the english pronounciation? Which letters and pronounciations from other languages can be used as well?
EXAMPLE:
"Sylphaej" is the name of the language of elve-like beings. Therefore it sounds very soft and has lots of v,j,y,ph and ae in it to make it special. It is spelled with almost no pause between the words to make it sound more elegant (like french). I was inspired by elvish ;)
"Khrast gen vren, khra di tieor, lar sen oargh" is a spell used by witches for dark magic. I wanted it to sound harsh and intimitating, so I used lots of k,t,g,m,r for the words. For a mystic sound I refered the pronounciation to north-european languages. I bareley used special letters like y,j,v and did'nt use any z,x,q,w or b sounds.

Step 2: Vocabulary and sentences
Once you know how it sounds like and which letters you want to use often, you can begin to mix some words together.
If you want to, you can start by looking at languages you like and recreate some words. Instead of "dear" I wrote "dhear". Or use words of other languages but give them other meanings. I used "Ó" as a sign for a person that is spoken to or about like in "Ó dhear est mar glej" which means "My blessing is yours", so the "Ó" stands before the "you"/"dhear".
EXAMPLE:
Remember the spell I wrote about before? Here are some other spell ideas I didn't gave a meaning yet:
"Ast chrat gen"
"Fej lost ghet nea"

Step 3: Grammar
Now you thought of some words and some sentences and gave them meanings. The easiest way to create rules for grammar is to look at the sentences you created before. Which order do the words have, do the verbs have to change in any way,... Then write the rules down. Then you can think of the other grammar your language needs.
EXAMPLE:
I got the sentence "Ó sien dhest mortem laest, ach khat mej, ach faer mej."
I know that "Ó" refers to the person spoken to and "mortem" is the name of a god (the god of death). The spell is used for calling ghosts and give them orders. So "sien" has to be "gost", "khat" can be "hear" and "mej" can be "me". So "ach khat mej" means something like "hear me". This has to be an order, so I need to have an imperative. To keep the language easy, I decide that the verbs don't change, so I need extra words to signalize it. That can be the "ach". Thus "ach faer mej" also is an order, which has to tell that the called ghost is supposed to follow the witche's orders. "faer" shall mean "follow" or "obey". Know I only have to decide what the words "dhest" and "laest" mean. The first part of the sentence has to call a ghost, a person that is dead. I thought of something like "ghost that death has in his claws" so I simplyfied it to fit two words: "ghost that death keeps".
So the first grammar rules that can be written down are:
The words' order is: subject, verb, object. And "ach" stands in front of a verb and signalizes the imperative.
Furthermore, I decided to invent other extra words that signalize the time, so the verbs don't have to be changed.

Always keep in mind that inventing a language is a process: you'll invent new words when you need them and new grammar if you need it by using the language.
I hope this will help you!