You will probably not get enough roots in a single season if you use only the wire tourniquet method. It may take it the better part of a year for the sap flow to be interrupted by the tight wire. Perhaps longer.
It would be better to remove a ring of bark at least as thick as the trunk is wide and dust the area with rooting hormone.
There are some excellent references for air layering in
bonsaiTALK Links under Techniques->Propagation->Layering. I think there is also a bit in the ***FAQ*** here.
If this is a particularly old and valuable tree and you haven't done an air layer before, it might be best to experiment first on another tree. It is very straightforward, but so is flying a kite.
Regards,
Matt