How do I install the blow-off barb for a FTSs / Bucket lid?

There are two O-rings included with the assembly.

One you use with installation and the other as a spare.

The one O-ring you'll use for installation goes on the top side of the lid between the barb and the lid, and the locking nut goes on the underside (beer side) of the lid.  No O-rings are used down there.

Please see the site listing HERE for some pics of the installed assembly.