Where and how deo I "attach" a file to website properties

My mind is in a turmoil as I have this particular nice scrolling application already running on 2-3 blue voda sites but for the life of me can’t seem to add another. I am missing a step outlined by its creator ” The nextfade.js isn’t attached”(to website properties). The sites I have it on are 100% OK but I seem to have forgotten how to duplicate the action.

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Where and how deo I "attach" a file to website properties