It’s about 40yrs since I studied electronics and things have moved on a pace since then so I hope this will still be valid.
Led’s are a dc devices with +ve and 0v connections, so a step down transformer on its own wouldn’t be sufficient as the lower voltage output from the transformer would still be ac.
You would now need to convert it to dc by rectifying it with a bridge rectifier. Something in the order of a 9v ac output from the transformer, when rectified, would give approx 12v dc.
Before connecting the led to the dc output you would need to know the forward voltage and current of youre led in order to calculate the correct size of resistor to protect it (by connecting the resistor in series with the +ve connection of the led to limit the current that would flow through it)
Hope it helps and is still current