Without knowing which PSU or which motherboard, generally speaking there will be two places that the PSU connects to the motherboard.
One will be a big fat connector that is 2 rows of 12 pins. You will have either a connector with 2 rows of 10 and another one on the same wire with 2 rows of 2 that goes into this or one with 2 rows of 12. Most likely the former sort.
If your PSU only has one with 2x10 and there is no other connector on the same wire, you should get a different PSU.
Second, there will be a port close to where the CPU is that is either 2x2 or 2x4. There will be a connector on the PSU that is either 2 sets of 2x2 on the same wire (most likely) or a single connector that is 2x4 (less likely) or a single connector that is 2x2 (still less likely). That is where you plug that in.
If either one of those connectors doesn't have all its pins filled, then you need a different PSU. Old PSUs have smaller connectors and might not fill up all the slots on a newer computer and that could damage hardware if you leave them without all the slots filled.