Constructing Śrī yantra is considered very difficult. There are several descriptions in the classical literature but most either produce inferior results or are incomplete or exceedingly obscure. The best analysis and practical guide to construct a correct "classical" yantra I've found is Gerard Huet's paper on the construction of Srī yantra. His construction is not geometrical but produced as solution to a set of equations with four free parameters giving various forms of "correct" yantras. He has even hand coded postscript code for experimenting with the parameters (very neat!).
I would still like to be able to find a geometrical construction
for this yantra. Check back in a few years!