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Run a single point at the STO-3G optimised geometry using a double zeta basis set(e.g. 4-31G).

CAS(6,6)/4-31G
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Now project the 4-31G wavefunction onto a larger basis set.

Run a single point energy calculation(again at this geometry) using a double zeta plus polarisation basis set(e.g. 6-31G*).

It is important to do the above in two steps rather than projecting a STO-3G wavefunction straight onto a 6-31G* basis. This is too much change in the wavefunction for one step. A good rule of thumb is to project up one step at a time(either by increasing the split valence level or by increasing the highest angular momentum functions included in the basis set) e.g. STO-3G  4-31G  6-31G*  6-311G*  6-311G** etc.