It's basically a ritual done solely on paper, in writing. It is based on self-hypnosis. An actual piece of paper is preferable to Microsoft Word. Start by describing your current environment and what you're doing. Write always in first person, present tense. Then go on describing how you're leaving and finding some stairs somewhere that descend into darkness. Describe how you descend on those stairs for quite some time, then stumble upon a door. "Knock and it shall be opened unto you." Start writing whatever comes to you from that point, without thinking, regarding what you find beyond the door. It might be a whole other world or a corridor or whatever. Or maybe a person. It's up to your subconscious. If it's an entire world, you may meet people, animals, all kinds of weird beings. They are all aspects of your subconscious. Try to understand them and ask them questions. If you only meet a person beyond that door, write your dialogue with him/her.
When you finish your journey, describe how you are going up the same stairs until you get back to your desk, where you are writing the whole thing. Don't forget to close the door
EDIT: Descending on the stairs is basically descending into your subconscious. But you can also climb. I only tried climbing once and it was extremely weird and intriguing. I'm guessing you go into your superconscious or overmind or something like that, but I would advise knowing your subconscious first, before your superconscious.