Use all of the original box and packaging materials that the unexposed film came in.
Put the film back in just as it was originally packaged (minus interleave papers).
It's wise to use the correct box for the type of film (Acros in an Acros box, FP4+
in an FP4+ box, etc.) but not mandatory.
Put a business card on top of the box so the lab won't get it mixed up, and use
clear packing tape to seal the box, including holding the business card in place.
Give them a written description of exactly what's in the box, including type and
number of sheets of film (only one type per box) so they know what to expect.
I normally put the film box and two copies of the Purchase Order/description
document in a large zip-lock bag to keep them together.
They'll normally return all the boxes and packing to you with the processed film,
but make sure you request that specifically just in case.