where do i put code for editing layout

i am a really slow person, and i cannot for the life of me figure out where i'm supposed to go to paste a layout code to edit my layout. please help im so exhausted writing this i need help figuring this out. 

edit: currently editing it. i thought my little profile coding knowledge from rentry would help and it is NOT. i cant see an example of what my page looks like so i dont know what is what</3 really should have learned this stuff before trying to do it.