(You may not need to do this on low-end cards, because the PCIe slot can deliver 75 watts on its own).
Push your graphics card into the PCIe X16 slot on the motherboard (it’s the long one closest to the center / top of the board). If your graphics card has PCIe power connectors on its front edge, you'll also need to connect those from the power supply to the card. (You may not need to do this on low-end cards, because the PCIe slot can deliver 75 watts on its own).