The exact proportions and relative size of the individual triangles in this yantra varies with the source. The construction used here uses two circles with a central equilateral Śakti (downward pointing) triangle inscribed in the inner circle. The large vahni (upward pointing) triangle circumscribes this circle and is in turn inscribed in the outer circle. The innermost triangle is extended upwards until it intersects the outer circle to produce the upper of the two larger Śakti triangles. By extending the base of the innermost triangle until it intersects the outer circle and connecting these points with the lowermost point in the outer circle we get the second large Śakti triangle.
The observant student may notice that nava-yoni corresponds to the two (three with the bindu) innermost chakras of the famous Śrī yantra. In theory it should be possible to use some variation of nava-yoni yantra as a starting point for constructing Śrī yantra outwards. However, I've never seen such a construction that works. It would take both a some very special form of nava-yoni (this one will not work) and some additional mechanism to determine further underdetermined points in the construction.