I consider it a largely subconscious process, actually. The spirits that I come to be familiar with, have always been a part of me, but they're the wiser parts that my conscious mind cannot contain. That's they they're sub-conscious.
The way to "create" them, or at least the way I have, is to set up particular spaces, times, and actionable spheres by which I can lower my natural inhibitions.
The space and time is for meditation. My imagination becomes the actionable sphere, or one of them. So, I imagine going into a meadow or clearing in the woods. I set out the intention for my familiar to meet me there, and I just sit and wait until it happens.
Another actionable sphere is in the overlay of the real world. If there's a not-so-friendly familiar, I still have to force myself to act normal instead of freaking out and shouting magic words to banish the bad thing. But I'm only sensing that thing because my mind's inhibitions are lowered. See? So, you can do the same with familiars. Sometimes they'll be in the same room with me while I'm talking to a real-life person and, and other times I'm doing a real-life thing while at the same time balancing a daydream of being in my meadow and interacting with my familiars there. It depends on where they want to be. They've never been in both at the same time where I have to figure out which one is "real" or something, but because they're created from your subconscious, I do believe that this can cause some erratic personality.
Or, read this:
I hope this helps!