This is a large and very complex subject- but to get started
- Start by chosing a domain of sounds you are willing to use. Some languages have oral contortions that many people find difficult to reproduce. You could also include gestures and facial expressions to suppliment the oral part.
- Invent nouns for very basic things. A noun is the name of something such as: man, rock, water, food.
- Decide if nouns carry gender (generally or only in exceptional cases). In french all nouns are either male or female.
- Invent adjectives (words that describe nouns). Decide if these precede nouns (as in English) or follow the nouns (as in French). Decide if they carry their own gender, are influenced by the gender of the nouns they modify, or need to be reconciled by prefixes, suffixes, marker words or adjoiner words.
- Come up with a rule from making compound words out of what you have.
- Extend you adjectives to include numbers. You do not have to use base ten. Base five is perfectly legitimate, as if base twenty. Base twelve has been used as has base 60.
- From number invent articles. In English the number one gave rise to the definite ariticles: a, an, one; the number three gave rise to the indefinite articles the, these, those, this, that. (you could make articles using the number two: interesting)
- Decide if the nouns or the adjectives carry the responsibility for purality. And then decide how plurality is to be expressed. In English we simply add an s to the noun (in most cases). In French we usually drop the end letter from all nouns so when we add an s we now pronounce the last letter. In Tagalog we do not modify noun but alter the article: THE CAT-Ang pusa THE CATS-Ang mga pusa. In french both nouns and articles carry plurality. THE BOY -- le garcon THE BOYS -- les gracons.
- Decide wether articles precede or follow nouns. In English both articles and adjectives precede the noun. The green house. In French the adjective follow the noun. La maison vert. (the house green). You could have any combination you like: green the house, green house the, house green the.
And lots lots more. The subject is vast. propositions, adverbs, conjuctives and on and on. If your going to invent a language make it a project of many months or it will overwhelm you. Do a little bit more every day and practice.