Here are some soldering tips...
I use a scalpel sharp utility blade knife to scrape mild oxidation off stripped copper strands. I lay small wire atop a finger the scrape the blade sideways perpendicular to the wire strands. I avoid the blade thing if I had the hiccups.
Cold solder mistakes are created by oxidized copper and using hardware store solder. Terminals actually see a "Blush" race across it when the tin or copper gets hot enough.
Hardware store plumbing PASTE not liquid flux can be used. But not frequently. It simply eats the solder gun 10x faster than rosin. But it does strip off nasty corrosion. For an emergency repair to old wire oxidized to dark purple, I've scraped the exposed strands shiny with a blade then used hardware store paste flux. Don't expect the wire to last forever.
I cannot think of a better way to learn to solder than to get ahold of some electronic 60/40 solder, and small tin of hardware store paste flux and complete trial soldering jobs on a bench. It happens fast you can see each soldered strand of wire. The solder flows like water and doing this is 10 times easier than you think when you have the right stuff.
