Yantra has the root Yam + Tra, which is Yam is to hold and tra is the instrument.
So a Yantra is nothing but a container or machine which holds.
A Yantra will contain triangles, squares, circles and other geometrical shapes to contain. The symmetric of these shapes makes it a machine or instrument.
Example : Shatkona or Shadakshara Yantra of Lord Murugan, which is Star of David in Jewish traditions.
To Energize a Yantra one needs the Mantra and to be recited with Prana Energy.
To consecrate the Yantra, one needs Tantra, which is Initiation, Pujas and Time.
To customize to one’s own use, the horoscope details can be used to choose the time to recite daily.
Without customization, if one does puja as above to the yantra, it will be for the benefit of all, which is being done in temples, by installing the yantra under the presiding deity in Tamil Nadu temples.