The fact that I made it, or acquired it from someone else through trade.
>Or more generally, how do you go from a world with no property rights, to one where everything has a defined owner, in a way that is simple and obviously fair?
The homestead principle is based on the notion that if something is unclaimed, and someone adds value to it, they should be able to enjoy the full benefit of that value-added thing.
The only 'artificial' private property is scarce natural resources, most importantly land, and that is because much if not most of its value comes from its natural form, rather than anything added to it by its original homesteader, and it is this class of property which society has a moral right to create rules/taxes on.
With respect to this latter class of property, once the rules on its use have been established (say a rule asserting that a property title owner must pay a rent equal to 1% of the value of their land to the government each year) anything the title owner earns in accordance with the rules (e.g. anything they earn on top of the 1% tax) is theirs by right.