Step 6 is definitely the most challenging, and may take a lot of experimentation and getting used to the feel of it to get it right. The good news is that it’s literally impossible to make it taste bad at this stage, you’re just controlling the texture by encouraging the sugar to crystallize in a particular way. If you stir for too long the fudge will be harder and crumblier; if you don’t stir long enough it will be softer and grainier. Either way, delicious!
The temperature is important because it controls texture the sugar crystals will ultimately form. Too cool and your fudge will be chewy, too hot and it will be very hard and likely the fat will burn. Sugar goes through something like phase transitions, so it will spend a long time just below 240, then spend a little while at 240 before heating up again. There are lots of important sugar temperatures, but 240 is known as “soft ball,” which means that, if cooled suddenly, the sugar can be shaped into a soft ball!
If you don’t have a thermometer that goes high enough it is possible to tell the right temperature without a thermometer. If you drop a small amount of the liquid into a bowl of ice water, you should be able to shape it into a soft squishy ball. This is often more reliable than a thermometer, but takes some experience! Other clues include: you have been stirring for a long time, you can smell a delicious sugary buttery smell, and the bubbling liquid is a slightly darker shade of gold than it used to be.
Golden syrup is an unusual ingredient in the States, it’s a bit like a cross between honey and molasses. It can be very hard to find in a store! The only brand I know is Lyle’s Golden Syrup, it comes in a green can or a squeeze bottle. Look in the baking aisle, the maple syrup spot, or if your grocery store has an “international” aisle with stuff from Britain, it could be there. It’s also sold by Amazon
I've substituted vanilla bean paste for the extract and it's just as delicious