To clarify, you can’t die of botulism from a dodgy jar of marmalade; leave that to the pickled cucumbers (although apparently they won’t kill you either, see the marvellous Sandor Katz). Pretty much the worst thing that can happen is mould. To avoid this, you want clean, hot jars, hot marmalade and sufficiently high levels of sugar and acid to inhibit the growth of bacteria. 212F is enough to kill off all the nasty buggers. An instant-read thermometer is a wise investment here; It doesn’t have to be hugely expensive and it’s a very handy piece of kit, good also for making fresh cheese, checking roasted meats and deep frying.
Although it’s tempting when you see the mountain of sugar sitting in the saucepan surrounded by a moat of juice, don’t skimp on the amount called for. The sweetness will mellow over time. I put my jars and lids (if the seal comes separately always use a new one, as it does degrade) in the dishwasher and then in a medium hot oven for ten minutes just before filling, and then again after sealing. If you’re unsure about whether to hand this out to friends, or if the lid ‘pops’ as the marmalade cools, just keep the jars in the fridge and eat within a year. Otherwise, store in a cool, dark place.