A portal opens in the mundane world. A portal opens for you every night when you enter the dreamtime, and death awaits as a magical doorway for the integrated spirit, but you may access portals in all sorts of other ways as well. Empowered imagination can open portals into other worlds. For example, both the writer and the appreciative reader of a great fantasy work are able to step through a portal and experience an alternative reality. One of the greatest magical portals is when a soulmate grants you access to their inner world. Oracles are designed to open portals into the plane of inner vision. Portals may open in the midst of the haste and bustle of the mundane world. Sometimes we can recognize the portal by its numinosity; it lights up in our mind's eye with an uncanny significance. Numinous objects are always worth investigating, but some should be approached with caution. The most unreliable numinous object is a person of great physical beauty because we will tend to literalize that person's numinosity and assume that the uncanny feeling of significance derives from the beautiful person and should be worked out interpersonally. This can be a dangerous delusion because this sort of numinosity is mostly intrapsychic and has more to do with us than with the person sparkling in our mind's eye. (See Stop the Hottie!) Other times the portal opening goes unrecognized because it comes as a seeming accident, as an irritating, annoying, disturbing, unsettling, unexpected event, and it takes some intuitive skill to recognize the accident as a window of opportunity. Windows of opportunity are opening and closing all the time. Stay alert and present in the moment to recognize those key portal openings that further your quest and step through them. Be wary so you don't get sucked into a portal that will sidetrack you. Recognize the portals as they open and close in your world, and make a conscious choice about entering or passing them by.
See: A Guide to the Perplexed Interdimensional Traveler The Path of the Numinous. Examples of Portal Experiences: Corridor Shred to Black -- Salvia Blue Moon Apocalypse Journey to the Lower Astral -- A Role Playing Game My epic story related to portals: Parallel Journeys