The only way 'easily' to do it with a DC supply is ..to use a resistor in series.
You first need to find resistance of your LED (usually small), if its quoted fine, if not you will need a ammeter. Use two pencil cells(in series) and an ammeter with your LED , to measure current, divide (3V / current) to get resistance.
Now since you require about 9 Volts to dissipate in your resistor,...
you will need a resistor 3 times the resistance of your LED.
Quoted ampere (1.5A), is usually a max you can draw from that 12V power supply you can ignore that.
You can also try guess work, but might burn few of your LED's