Connecting led fairy lights together powered by ac mains
Four 3 Volt LEDs connected in series will work diretly from your 12 Volt supply, with no extra components.  One such group will draw 15 mA.

You can connect up to 66 such sets in parallel to the same 12 Volt 1 Amp power supply.

Correction:
I must have been thinking of incandescent lamps when I wrote the first sentence.

LEDs must always be used with a current-limiting resistor, so you want a group of three LEDs, plus a 200 Ohm resistor to operate from 12 Volts.  One group will draw 15 mA.
(second sentence is OK)