A 12 volt ac transformer will result in approximately 17 volts dc when rectified and filtered. A full wave bridge and appropriately sized capacitor followed by a switching buck regulator module is one way to achieve your goal. But, by the time you’re done with sourcing, fitting and mounting the additional parts required, it’s probably easier to purchase a dc output wall wort to power the led’s.
Do you know what current at 12vdc is required for the led’s? Can they be driven with a constant voltage? An led driver is usually a constant current device.
That’s the missing information at present for a more informed answer.