Question about rep ranges and when to add weight?

I'm doing DeFranco's WS4SB3. When I'm given a rep range (i.e. Rear Delt Fly 3x8-12)

When do I move up the weight? When I do 12 for 3 sets across? When I make it within 8-12 for all three sets then work my way back up? Should it be easy or almost failing?

I feel like such a n00b asking this, and its probably common sense but I'd like a clear explanation!

Thanks

edit: shit it's been a year?? woah....