All that needs is some flux and more solder per joint.
Alternatively (and what I personally prefer), you can tin both ends and then join them together. This works because most solder has flux inside of it (specifically, solder is a hollow tube with flux inside).
In chemistry, the rule is called "like likes like" or "like dissolves like", which means that similar materials like to join together, and dissimilar materials don't.
Oxide layers on metals prevent other metals (like solder) from joining to it. What flux does is it chemically cleans the surface as it heats up, removing that oxide layer. This then makes it easier for solder to join to metals such as copper.
A common, more obvious form of an oxide layer on metals is a rust (iron oxide) layer on iron. That rust has very different chemical and physical properties to iron, as it is now a fundamentally different material.
My goto flux is knockoff Kingbo RMA-218 from China (the whiter stuff is much better than the yellow stuff, and is thankfully the more common variant).