bait
This is dangerous advice. The range in dosage between species of psychedelic fungus varies by around 10x, and a lot of them look fairly similar.
Basically it perturbs the brain's pattern recognition systems, and causes them to look much more strongly and insistently for patterns and regularities in the input. Therefore, to get the best effect, one should feed in highly irregular images, such as the messy, fractal-ish geometry of nature.
When you look at tree bark or grass or clouds on psilocybin, your brain tries furiously to see a pattern or regularity in it, and as a result your perception of it constantly shifts and rearranges.
When you stay indoors and look at manmade things with human-conceived geometric shapes, the patterns are already there and your brain sees them immediately. At best this is boring, and at worst it can cause you to become fixated in a way that is quite uncomfortable.
This isn't the most eloquently worded explanation but I'm sure most people here can tell what I'm talking about. Psychedelics have a lot to teach us about the workings of perceptual cognition as well as about more spiritual matters.
Theyre not biased towards positive or negative experiences, just towards incredibly earnest depictions of the drug experience. People don't bother to write up the boring shit.
When you are really experienced and no longer have any anxiety about what lurks inside your subconscious, only then is it advisable to trip alone.
Bear in mind that if you are a generally grounded and lucid person, it is entirely possible to experience things as extreme as this while never once thinking that it's really happening.
Sitting in the middle of a university campus watching it being torn apart by terrifying non-euclidian-shaped monsters from beyond the sky is really fucking cool as long as you never forget that it's just a drug experience. It's like being in the middle of the best action scifi movie ever.
Don't rely on this happening to you though ;)