J Adam G:
But as for the specific techniques of yantra, the things that set yantra apart from other visual meditation objects, I don't know what to tell you. The best thing would be to find a teacher who knows about yantra, and the next best thing would probably be a good book on the subject.
Yantras usually have syllables that accompany them, and these contain an incredible wealth of knowledge that can be aquired through contemplation of the yantra. Not for the head, but for the heart. This kind of meditation then can look like calligraphy as you can draw the thing hundreds of times and deepen your understanding every time. The knowledge there is very dense, they're masterpieces, don't know if this approach can work without a teacher. I've looked for, but never seen a useful book on the devanagari written in / translated into english, and around the shri yantra you can place all the syllables. Each syllable has its meaning, as has each triangle. Hindu yantras (I don't know about mandalas) were made for preserving knowledge about reality's working and for reshaping your mind, not only for reaching a jhana.