Dimming 6W (24V DC 0.25A) LEDs using Arduino
I would suggest using the CAT4101 constant current sink PWM LED driver.  It can handle up to 1A at up to 25V and deals with the constant current regulation for you - just provide at least the 24V you need and enough current capacity and connect the correctly calculated current setting resistor and the CAT4101 will do the rest for you.  You can then control it through the PWM input pin direct from the Arduino.