All ancient cultures had sacred geometric designs representative of their Gods, which had a mantra (or sacred sound vibration) that corresponded to it. In the Vedic culture, we find much power and energy said to be held within sacred geometric symbols.
Yantras are not "lucky charms" to be worn or displayed as icons of power in and of themselves. Much specific knowledge and ability must go into their preparation and much dedication and worship is required to achieve the desired effects.
Yantra means a "talisman", or "instrument" or "Amulet" or "Kavach" which, if prepared and created by a qualified person as well as utilized under his specific instructions for fruitful results, will help to gain the objector objects of desire or ambition. Although it would not be a difficult task for most of us to copy the form of a Yantra, it would not have the desired effect. Furthermore, to be perfectly honest, it would be all but useless if not created by a qualified person and then "infused" with the specific energy via the medium of mantra. Otherwise, it becomes just an interesting form or picture to look at, but has no real effect on time or circumstances within life.
Yantras should always be used on the level. If drawn on paper, the colors preferable are red, orange, yellow or a combination of these. A Yantra without bija mantras is dead. They can be drawn to whatever size is required. In worship they should be placed level on a pedestal or 'pitha'. Before any Yantra is a suitable object for puja, it must be given life (pranapratishta).
A Yantra is essential in every field of your life, to attain success in your profession, to acquire wealth, to win in court cases, to ward off diseases and to progress in meditation. By having a Yantra and keeping it in a sacred place in your house, gazing it everyday can fulfill your desires and help in attaining your goals.
It is auspicious to see the Yantra daily in the morning and one should lit incense / dhoop or lamp with ghee before the Yantra every morning. By doing this, native is blessed with success and wealth.
Yantra is the use of certain external objects, symbols or some mechanical means to worship the divine. The act of folding of hands in front of the deity is but a kind of yantra only. The manner in which a fireplace is built for the performance of some Vedic sacrifice, the method in which the place is prepared and the materials (sambhra) are assembled, the manner in which the oblations are poured into the fire, the way the priests sit around the altar, and in fact the very act of chanting of the mantras with mechanical precision form part of yantric worship only.
The very design of the temple as an outer symbol of the existence of the Divine on the material plane, the act of visiting the temple, circling around the temple, entering the temple, the lighting of the lamps in front of the divine, the decorations and the ornamentation so characteristic of Hindu temples and places of worship, the manner in which the images are built and installed, the lighting of the lamps, the offerings, the method of worship, the partaking of prasad, and in short any practice that is mechanical, symbolic and ritualistic to a degree, form part of this approach only.