Welcome to the Nexus!
Personally, I like to travel to DMT land while in the comfort of my own bed, with low to moderate lighting, blankets/pillows/anything tremendously soft, no music, something to set my smoking/vaping device on, and a bottle of water. I also like to light some incense and I'll walk around my room, ringing my Tibetan singing bowl to help clear any negative energy (my own form of smudging, I suppose). After I take my dose and safely place the device down, I like to lie back on the bed and let the experience wash over me. That takes care of the physical setting,for me typically.
I find DMT to be quite intense, so my mental prep usually takes a lot longer than setting up the room. I like to sit quietly and focus on my breathing, calming myself until I am ready to take the hit(s). Sometimes, I accompany the breathing with a reassuring mantra of sorts, designed to help overcome any pre-flight jitters.
Something else to consider would be whether or not to have a sitter. I like to talk with someone after the experience is over but I don't usually have a sitter (my girlfriend) in the actual room with me during the experience itself. Ultimately, you will have to decide what feels right for you, of course!
You are right when you say there is no way to truly prepare, but I think that is partly what makes the DMT flash so unique and beautiful.
Anyway, that touches on what I do to prepare. Have a good one!
PEACE
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