Fit the processor and the fan provided with it. Take care of the cooling agent (Thermal paste) applied on the processor which connects to the fan.
Connect the ram and ssd to the ram and sata ports.
Connect the power supply to the motherboard and the peripherals.
Connect the gpu’s to the risers and mount the risers on the aluminium case. Connect the risers to the motherboard and the power supply. Depending upon the power supply of the gpu this can vary.
Connect screen, keyboard and mouse (peripherals to the) motherboard.
Connect the power starter cord to the mentioned i/o power sockets on the motherboard. Refer to the circuit diagram of the motherboard for the same.
Mount the fans on the aluminium case adjacent to the gpu’s for cooling.