Article 1796 of Mexico’s Federal Civil Code begins: “Los contratos se perfeccionan por el mero consentimiento, excepto aquellos que deben revestir una forma establecida por la ley” (Contracts are perfected by mere consent, except those that must take a form established by law). Article 1803 says that consent is express when it is given verbally, in writing, by electronic means or by unmistakable signs.
Article 1832 adds that in civil contracts each party is bound in the manner and on the terms in which it appears that party wished to be bound. No particular formalities are needed for the contract to be valid, except in the cases the law expressly names.