i'm not trying to sound graphic but poison overall is a very, very painful way to go, if it even manages to kill you in the first place. but the yew tree in specific- i mean, it'd be a culmination of every system in your body having a horrible time all at once. you'd likely end up with several things happening together, including but not limited to: consistent bowel movements that you cannot control, extreme hallucinations, neurological damage that could cause seizures in the moment, all sorts of reactions from your heart which are by themselves terrifying and painful in their own right, constant vomiting (and i need you to understand that what happens at the lower half of the body can happen to the upper half. you could, quite literally, vomit excrement), and more
i'm not comfortable giving tips on how to CTB, so i am sorry but i can't give you a better alternative. i do, however, need you to understand that what you're considering is perhaps one of the worst things you could do to yourself. you will suffer in a way you cannot possibly imagine. please do not try this method