Short answer. Three ways. By law, where a statute itself confers the enjoyment; by the will of private persons, either in a deed taking effect during life or in a last will; and by prescription, where someone has enjoyed the property long enough for the right to ripen in his favour.
What the law says
Usufruct is constituted by law, by the will of private persons expressed in acts inter vivos or in a last will and testament, and by prescription.
Civil Code, Article 563 — How Usufruct Is Created. Read the full provision →
By law, without anyone agreeing to it
A legal usufruct arises because a statute says so, not because the owner chose to grant one. Nobody signs anything; the enjoyment attaches to a status the law recognises. The consequence worth knowing is that a usufruct of this kind cannot be negotiated away in the ordinary manner of a contract, and its terms are found by reading the provision that created it rather than any document between the parties. If you believe you hold such a right, the first task is to identify the provision, because everything about the right's scope and duration comes from there.
By the will of private persons — the common route
Almost every usufruct met in practice comes from this branch. The article recognises two forms of it: acts inter vivos, meaning an instrument that operates while the grantor is alive, typically a deed of donation or a sale reserving the usufruct to the seller; and a last will and testament, where the enjoyment is left to one person and the naked ownership to another. The distinction is not cosmetic. A usufruct created by will takes effect only through succession and stands or falls with the will's validity, while one created inter vivos operates on its own terms from the moment the instrument does.
By prescription, which is rarer than it sounds
The third route needs no grantor at all: the right is acquired by enjoying the property in the manner of a usufructuary, openly and continuously, for the period the law requires. It is genuinely available, but it is the branch most often claimed by people who cannot in fact establish it. Enjoyment that was permitted by the owner as a favour builds nothing, because it is not adverse; nor does an occupation that was interrupted, or one nobody can date. A prescriptive claim lives or dies on evidence of how the property was used, by whom, and from precisely when.
Getting the creation right at the start
Because a usufruct binds property for a long time and often for a lifetime, the instrument creating it should say plainly what many do not: which property, in favour of whom, for how long or until what event, and what is to happen to improvements at the end. Where the property is registered land, ask separately whether the right has been annotated on the certificate of title, since a right that exists between the parties and a right that is visible to the rest of the world are not the same thing when the property changes hands.