A yantra intended for actual ritual use, rather than as a printed poster or a piece of jewelry, is normally engraved on copper, silver or another metal plate by a craftsman working to precise geometric instructions drawn from tantric texts, with mantra syllables inscribed in a specified script at specified points, a construction where the proportions and placement are the substance of the object rather than decoration around it.
Consecration and transmission
Once made, such a yantra is understood in most traditions to require consecration, prana pratishtha, by a qualified priest or tantric practitioner before it is considered active rather than simply a correctly drawn diagram, and in several tantric lineages a yantra tied to a specific deity practice, sadhana, is transmitted from a guru to an initiated disciple as part of that practice rather than assembled independently by a newcomer from a book.
What this atlas will and will not say
This is a genuine restriction, not a formality: several yantra forms, including some of the best known Shri Yantra variants used in serious tantric practice, are treated by their own tradition as closed to uninitiated reproduction and use. The commercial yantras sold widely as posters and pendants are real objects and are not secret, but they are also generally understood within the tradition as devotional rather than as the fully activated ritual implement, and this entry describes that distinction rather than supplying construction or consecration instructions for the restricted form.