But how about flux? It's not necessary for freshly stripped wire?
Flux is always necessary when soldering, but it must be the right kind. Never use plumbers flux or 50/50 tin/lead solder. You should use a tin/lead alloy solder with a built-in
non-corrosive rosin flux core, 60% tin and 40% lead. You can also use a eutectic alloy of 63% tin and 37% lead, again with a rosin flux core.
Eutectic tin/lead alloys have a very narrow "plastic" state between liquid state and solid state as a function of temperature. They are somewhat easier for beginners to master the proper soldering technique.
Tin the stripped wire first after twisting the strands enough to keep them from separating. Then heat the metal to which you are trying to attach the wire and apply a thin coating of solder. Make sure you heat the metal enough for the solder to flow, not "blob" up into a ball. Then apply the tinned wire to the metal while still heating the metal with your solder gun. Add a little more solder to what is already there so as to cover the bottom and sides of the tinned wire. A third hand is useful for this: one hand to hold the solder gun, a second hand to hold the wire, and the third hand to apply the solder. Now here is the tricky part: remove the solder gun from this joint and hold everything perfectly still while the solder cools and solidifies.
If you are successful, the cooled solid joint will be smooth and shiny. If the pieces moved while the solder cooled the joint will look gray and pebble-like: this is a "cold" solder joint. You will have to re-heat and try again. It sometimes helps to strip a scrap piece of wire and separate out just one of the strands to use as a "tie down" to hold the tinned wire against the metal. If you try that, you will be heating both the wire and the metal it will be attached to at the same time, so it may take longer to get hot enough to melt the solder because the tinned copper wire will absorb some of the heat. Upside is now you only need two hands.