[rotorwrench]

Tinned copper terminals are better but sourcing some of these in the original bullet pin and socket type are not as easy as they once were. Soldering requires a flux so that's where the resin core comes in. I use a good quality soldering iron and the modern tin/silver/copper solder with resin core for electricals. Use tin/lead with resin if you want to but I have good results with the modern stuff. I trim and prep the ends of the wire and then tin them on the ends with solder before joining to a terminal end. I keep a wet rag or sponge close by to cool and clean the soldering iron tip when it gets too hot or too loaded with solder. Place the tinned wire into the terminal then heat the terminal and apply enough solder to get both well joined. It doesn't take much with a pre-tinned wire end. If is a solder cup type terminal, just fill the cup when joining.

There are holding clamp fixture tools that will hold assemblies for you to keep your hands free to work. If you use them just make sure they aren't so close to the solder joint as to pull too much heat from the parts to be joined. I sometimes hold the solder between my teeth to free up my hands. What ever it takes to get the job done.