How to increase weight

rebprest

I've read a bunch of posts about this, but I still don't understand. looking at my curling (just for example), I can currently do 10 lbs for 5 sets of 5 lifts. I'd like to go up to 12, so should I start by going up in reps with 10 lbs, or do I make 1 of my sets 12 and 4 @ 10, adding another set at 12 each time? What is the method here?