The guides say different things because there are almost as many solutions as there are people, and there aren't very many "wrong" answers. My preferred method is to install the PS first, then assemble the MB, CPU, cooler and RAM outside the case and install the MB assembly. Hook up MB power leads, CPU fan, power switch lead, monitor, and keyboard and try to boot into UEFI/BIOS. If everything's cool, power down and move on to fans and OS drive. Front panel leads can be hooked up at any point from here on. I usually leave non-essential drives and expansion cards until last, including GPU if there's integrated video for boot test and (possibly) OS install. You may need to modify any suggested order if your case causes any component...