From the PSU you power the MOBO the GPUs and the risers too, at the top of the GPUs you plug the 6+2 pin PCIe connectors and power the risers with the other 6-pin connector, if you can reach to them, or with the 4-pin Molex connectors, max. 2 per cable!! If its possible don't use the Sata-6 pin adapters to power the risers or max. 1 per cable...