I regularly have a roof box on mine, and I've transported a 4 foot fish tank + stand before now without problems.
Getting something large onto the roof is tricky. With the roof box, I find it's easiest to balance the thing on my head and then slide it more or less level onto the roof of the car. If you're lifting something heavier than that, then it'll probably take two of you.
I use the cheap ratchet tie-downs that you can get from pound shops. The cheap ones do tend to rust and jam up after a while, but at that price, who cares?
I use one on each bar, so that it goes over the load, under the bar, over the load again, under the bar again, then hook the two ends together and tighten right up. If the item you're carrying is likely to be marked by the straps, then put some padding in there before tightening.
If you have more tie-downs, then you can also loop one or more around the load and the bars front to rear. That way, in the unlikely event that the load slips forwards or backwards from under the tightened straps, it can't actually come away from the bars. It's probably overkill, as the straps will effectively tighten their grip with any movement of the load, but if you really want to cover your backside, it does no harm.
If you're driving in bad weather, then you'll need to cover the load. In any case, if it's something of value, then I'd cover the leading edge to protect it from stone chips.