1 (the modern style): Put an S on the end of all of them, no matter what - "Buffy's hat", "Giles's hat" - except when they're an old historical or mythological figure - "Archimedes' hat".
2 (the traditional style): Leave out the S from names that end in S - "Giles' hat" - unless you judge that the pronunciation would emphasise both S's - "Joss's writing".
In this case, Joss is both a modern-day guy, and someone with the double-S pronunciation - so it would usually be "Joss's".
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1 (the modern style): Put an S on the end of all of them, no matter what - "Buffy's hat", "Giles's hat" - except when they're an old historical or mythological figure - "Archimedes' hat".
2 (the traditional style): Leave out the S from names that end in S - "Giles' hat" - unless you judge that the pronunciation would emphasise both S's - "Joss's writing".
In this case, Joss is both a modern-day guy, and someone with the double-S pronunciation - so it would usually be "Joss's".