Simple LED Dimming from Analogue Voltage Control
PWM modulation is the way to go with this as it saves wasting a lot of power in heat that an adjustable current source would generate. 

Building a PWM control can be done with many types of circuit designs but honestly the simplest to implement may be with using a microcontroller on a low cost board such as an Arduino. 

If you want to build your own circuit there are a plethora of circuit resources available on line. 

On the other hand if you want to keep your life as simple as possible at very low cost then consider visiting eBay and picking up a low cost adjustable module for a few bucks.