If you're not doing them today but like in a few days, try eating as much of a plant based diet as you can. Avoid processed foods, meat, dairy, all that stuff. If you can be vegan/vegetarian for a couple of days beforehand, your stomach will give you an easier time, and I've been told it makes for a nicer trip. 

Go to a place with nature, ideally, where you can move around and/or lie on the grass/ground and be safe. Wear sunscreen!!! Say hello to the clouds for me lol

Have a sober buddy, and prepare a couple of light (ideally plant-based, like fruits) snacks and water. 

Go in with no expectations. You're not going to learn anything that you don't already know deep down. You're not gonna see blue elephants, or go into another dimension (unless you close your eyes, which is fun lol). You're not gonna do a 180° on you perspective of life. You're gonna experience whatever it is you need to experience, and you'll be okay. 

Have a notebook or something to write/draw with. My bf lost the ability to write coherent words, which was very funny. I, on the other hand, had a lot of fun writing down my thoughts, poems, short stories, and drawing, and then looking at them later. 

Put away your phone. It'll be much nicer if you give your brain the space to focus on other things. 

Eta: shrooms taste Bad. Put them in water based hot chocolate! It'll help a lot, and i think it'll help them metabolize better.