The secret key for using Yantras in meditation is Resonance. The process of Resonance is established by mental focus on the image of the Yantra. As long as the mind is tuned into the specific mood associated to that Yantra, the energy flows, but when the Resonance is stopped, the energy disappears. 

Instructions for using a Yantra :

• The Yantras have to be purified and energized before using it.
• It energized during the nights of Maha Shivratri, Holi and Diwali are most effective and powerful.
• Lit dhoop or lamp with ghee before the Yantra every morning.
• Keep the yantra in a sacred place.
• Red, orange, yellow or a combination of these should be used to drawn it on paper.
• A Yantra without bija mantras is dead.
• Hang the Yantra on a wall facing North or East, placing the center of the Yantra at the level of your eyes. 
• Your posture should be accurate i.e your bak should be straight.
• Breath in through the nose and out through the mouth.
• Look into the center of the Yantra, trying to blink as rarely as possible; you don't want to look at the particular details of the Yantra, just keep your sight right in the center and observe the whole Yantra at once
  

One should do this exercise at least 15-30 minutes every day;after at least seven days of Yantra meditation you will be able to tap into the yantric energy and you will see the effective results of this exercise..