Your nvidia card should have had an adapter and instructions.
see this page:
I am building my first pc, I got the RTX 4070 and I have the power supply corsair RM850X and I realized that I don’t have a single PCIe cable, I only have the cables that have two PCIe cables while my 4070 only needs one, and my 4070 came with no adapter. I was wondering if I needed to buy an adapter or buy a single PCIe cable, or if I could plug just one of the two PCIe cables into the 4070, even though I doubt it would be enough power, I can only find single corsair PCIe cables online however https://nox.tips/
I got this,...