The big problem of soldering is the resources. If you don’t have the right materials, the right solder and the right flux, you’re going to end up botching the whole thing like I did.
It was fairly obvious that I was going to mess up. But hey, you know what they say: if you must fail, fail spectacularly!
Oh well, eventually I’ll have this 20×4 LCD set up and wired to the Elegoo Uno R3 (an Arduino/Genuino Uno clone). Unfortunately, I don’t have those easy-to-solder pins, which is why I have to do this ugly hack soldering the cables in. Hopefully the LCD doesn’t turn out to be destroyed by the heat.