Yantra means symbol of God. To do a contraction of a very huge thing (power or total divine knowledge) on a special material (like Copper, Gold, Silver) or to keep control on that particular thing is called as Yantra. It is an emotional invocation, if one wants to worship certain God in a special way then the God is resolved in that Yantra. The Yantra is drawn in the geometric shapes like square, triangle or circle. If one doesn't have an idol which one wishes to worship than one can worship the Yantra of that particular God eg: Ganesha Yantra.
Detail scientific information and procedures about Ganesh Yantra is given in Meru Tantra, Sharadatilak, Shakta Pramod, Mantramaharnav, Mantra Mahodadhi. Some of the information available in Ganesh Purva -Tapiny Upanishad and Ganesh Purana. There are four types of Yantras, namely Bhu-prushtha, Kurma-prushtha, Padma-prushtha and Meru-prushtha.
The lines on Bhu-prushtha are deeply engraved on the Yantra, where as lines on Kurma-prushtha and Padma-prushtha are on the Yantra, but not deep. Three different types of Aavahan(invocation) and Prana Pratishtha(consecration of an idol) of Yantra can be Achala(non-movable/stable), Chala(can be moved) and Dharana(can wear). In Achala Pran Pratishtha, like deity one can not move the Yantra from the fixed place. In Chala Pran Pratishtha, one can move the Yantra from one place to another along with the respective God. In Dharana Pran Pratishtha, the Yantra should always be on the body and one can take away the Yantra from the body only for worshipping of the Yantra.
According to the experts, the weight of the metalic Yantra is very specific. If the weight is not proper then it affects the worshipping. Worshipping does not affect if one has Yantra on any crystal.
The Yantra's are more effective and powerful when engraved on the Praval(coral), Padmarag(ruby), Neelmani(sapphire), Vaidurya(cat-eye) than the Yantra carved on Copper or drawn on silk cloth/paper and very much benificial to the worshipper. If Yantra is engraved on Copper it acts 100 times faster, on Silver it acts 10,000 times faster, on Gold or Crystal it acts million times faster compared with the silk cloth/paper.
The practice of worshipping Yantra is to keep Yantra in a box or a container.
It is advisable to worship any kind of Yantra under the supervision of an expert.