What do I need to add when I find 12v on the circuit board and want to add 5v?

You weren't very specific. You already have a 12V power port on the board, you now want a 5.0V voltage port.

If so, it is simple, depending on the power size of your 5V load, choose the appropriate LDO chip or fixed three-terminal voltage regulator can be achieved. For example: HT7550,LM7805 and so on.