Measuring current drawn on a 12V barrel plug
Yes, your description is correct.


cut one conductor of the disposable 12V power cord. NOT THE 110V or 230V CORD!
strip the insulation from the cut ends
set the multimeter to it's amps range (typically 10A)
connect the multimeter's probe leads to the COM and A terminals on the multimeter.
plug in the power supply to the wall outlet 
place the probes on the cut ends, making contact with the copper conductor
the multimeter will show the current
measure current under a variety of conditions, startup, normal, full brightness, etc.
crocodile/alligator clips make this easier


  
From Fluke 77 Series IV User's manual, 2006, Page 8 &amp; 2. - 
This meter meets CAT III and CAT IV IEC 61010 standards