Best way to make precision angled cuts

Hi all, I am building out my kitchen. My walls are not perfectly true and I’m installing finishing pieces on the ends to hide the cabinet seams. So it needs to be a very precise cut. Since it’s not exactly square and is off by like 1/4 inch or so what is the best way to make this cut precise? Would a saw guide be good to use? Let me know if I left out any details that might help.