I wrote a nice introduction to linkings in French here:
http://www.oegugin.com/2014/02/28/linkings-in-french/
Two of your examples are "liaison", and one is an "enchaînement". Those are different: you add a sound for a "liaison", but you just rearrange the sounds for an "enchaînement". Also, "liaisons" can be compulsory, optional, or forbidden, but "enchaînements" are always compulsory if you want to sound natural.
The first one ("un auteur") has a "liaison", because this N is usually silent. You add the N sound and pronounce it with "auteur".
The second one ("une auteure") is simply an "enchaînement" - the N moves over to be pronounced with "auteur" rather than stay with "une", which would sound awkward.
As pilko said, the vowels in "un" and "une" stay the way they were:
[ɛ̃]-nau-teur vs [y]-nau-teur
As for your last sentence ("jupes orange"), that's technically an optional "liaison", but it would sound weird to me. Like others have said, no "liaison" (that S stays silent). Here's what I found:
"Liaison facultative (mais conseillée) entre un nom au pluriel et l'adjectif qui le suit.
"Il raconte des histoires effrayantes.
"Ce sont des personnes élégantes."
http://www.francaisfacile.com/exercices/exercice-francais-2/exercice-francais-87708.php
Aha! Parce que c'est une couleur, c'est mieux d'éviter la liaison:
"Des rubans orange (la liaison zorange, bien que facultative, est parfois déconseillée afin d'éviter toute confusion avec le fruit ; autant privilégier l'adjectif orangé, dans ce cas)."
http://parler-francais.eklablog.com/adjectifs-de-couleur-a29368368