There are several important items/people you need to get the different endings.
Tips to Get Few Endings for Derek
You can receive this from Derek if he catches you in the desert, just tell him you’re scared and accept his offer. Drink this when your low hunger.
You can get Thomas by going up the hill at night, searching around, and going inside the cave. If you go in and you get attacked by Komodo and Dragon, then you went in too early. Stay outside on the hill 2-3 times until when you enter, you see them about to kill Thomas, then wait, grab Komodo, and (either choice is fine, though if you want the Wander the Desert ending (which requires 0 sanity), pick kill to lower your sanity. Then you’ll have Thomas.
Jaqueline can only be saved on the second day, so time is of the essence. You must have your canteen empty so that you can go the fissure, search around the area twice, and then find the poisoned water. Once you fill your canteen with the poisoned water, head back to the camp when it is safe and place it there. Then when you go back to the fissure, wander a bit till you see Jacqueline being attacked, you should be able to save her then (and also make sure the cave is available for her to join your group).
He is the last one to join your group, you should have Thomas and Jacqueline first if you want to get some of the endings with him, otherwise, he’s useless. But all you have to do is wait inside the cave until one of your members say they hear something, go look out, and see him scrawny butt outside. Save him, and now you have all members.
If you want the other death endings, just do dumb stuff like drink the water before Thomas does so you’re sacrificed instead. And if you go back to the camp at night, Derek will kill you in a really… unsavory way. Jack also will kill you automatically if he finds you in the camp at the day, or the fissure at the day. And if you get your insanity to 0, you’ll get the Wandering in the Desert ending.