Ah ok lol
Best option start out clean unplug everything.
Then simply plug the PSU into the motherboard and that's it. As long as the processor is in too it should power up.
so simply the system board with Processor and psu connected and nothing else. As Random said before listen for beeps. It would be worth doing what he suggested also by plugging monitor into the adapter connector build onto the system board. So you can see if it starts booting up.
Do the basics first then if it does get power one at a time connect everything else. Start with the memory next. Do the graphics card last.
I’m guessing at the early point I’ll want case power, front panel connector etc. in as well for that first test