How do you go about doing this with Sound Forge?

use limiter and set the threshold just about the db's of the hiss and make sure you set your attack and release on it correctly other wise it will cut in and out like a hard gate, or you can do the same wit a Exspander/gate if you set the settings right and compressing it a lil helps to mask it somtimes and then most of the time hiss is at the very top of the eq so cut the eq down just at the top a few db 3-6 or so should do the trick most of the time.

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