I always add supers to the top. That way, I don’t have to deal with storing boxes and risking robbing behavior from other bees. It also ensures the frames below are fully capped. Once that first super starts to be capped, that’s when you should add another super — even earlier if you have frames that already have comb. But if they are new frames with foundation, it will take them a while to fill that.
Some beekeepers prefer to pull the top super off and add a new one underneath it, and then place the full one back on top — the idea being that the bees don’t have to crawl as far up the hive. That works, but once you get three or four boxes, it becomes pretty cumbersome. I’ve had as many as five supers stacked, so I find it’s more practical to just keep adding to the top.
I also run a top entrance through the inner cover, which really helps with ventilation and traffic flow. The biggest impediment is the queen excluder. Once the bees start putting a decent amount of nectar in that first super, I pull it off immediately. If the queen moves up and sees the nectar and honey, she’ll head back down. But if there’s nothing up there, she will lay in your supers.
I think the nectar flow should last another week, the canola bloom is almost over and as dry as it is, weeds and other nectar producing plants will run out of energy. We all need to extract early due to the canola. And I will make a post on that shortly.