+ Does the stand-by light on the motherboard light up? If so, motherboard is connected to psu basically; but it can be that either a second connector is necessary for standby and suspend modes, or that e.g. the graphics card draws to much power from the pci-e bus. If the latter is the case lookout for necessary connectors to the gpu or additional gpu-power-connectors on the mobo which will provide the necessary initial power for the gpu via pci-e and then switch to directly connected additional power. - It is a Rx 580. Feed it!
+ If the fans at least initialize and the PC seemingly runs, but you just have no signal, either check the reference of the card for what connector inits first, or just start with the connector nearest to the pcie-e-slot | motherboard. Also you can check in the HUD of your monitor, how signaling should be processed. Fully digital EDD-ID which enables control by the os (that is yet not there ...) or classic pure display-signaling. Also you mostly have the option to override the automatic detection of signals and instead force a specific connector or hinder oing to standby.
+ Leave all LED- and Switch- -cables aside. Lookout for the pins for the powerswitch and shortcut them with a screwdriver instead, to power-up.
+ Use one DIMM only, in any but the first ram-bank-slot. If that does provoke a beep or likewise POST-event, e.g. signaled via standby-led on-board or the connected pw-led or hdd-led, check it. In most cases it will mean that either the CAS-setting could not be read or set correctly or it is angry that you didn't placed a DIMM in the first slot of the first bank.
+ Do not 'breadboard' or bridge the psu directly if you do not know what you are doing - and i mean - not at all understand what you do and haven't even a shed of an idea what can happen if you accidentally or forced bridge the wrong ones or shit happens and you break your mobo, simply because you can not clamp a cooler on a not mounted board - well you can - but it is nearly guaranteed that you compromise or completely destroy one or a bunch of layers. Motherboards are 3-dimensionally connected sandwiches. You will have no chance to 'see' that it is broken and it may be that it will work for a while until it doesn't. - These are not things to toy around even if you are like me the type of guy that sets up, for testing, a mobilee consisting of the parts connected, only holding together by gravity or precisely the redirected gravitational pull that fixates cable-connectors in the given slots. - And yes you need tons of components and a good amount of time to actually grasp the how and why while wondering if that is really happening. - Have fun. - Don't do it.
... just some thoughts added on top.
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Hope it helps. Maybe. Thanks for reading.