Before you can create custom links, you must make sure you are on a Tier II plan, and configure your DNS settings by following the steps in this help article.
Once that is set up, you can now customize the slug of your itinerary and lookbook links. To do this, simply click the Share button and then type in what you would like the link to appear:
** Note: make sure NOT to include any spaces or characters in the link you create as this will break the link! Ie, instead of itineraries.philwestsafaris.com/rachel's itinerary (which will not work) use itineraries.philwestsafaris.com/rachels-itinerary