There are a few kinds of solder. silver solder (for gullible audiophiles), acid core, and rosin core. You should use rosin core.
1. cut off about a 1" piece of wire then restrip the wire.
2. place the soldering guns tip under the speaker wire and hold it there for a minute or two.
3. touch the tip of the solder aginst the wire until it melts and then apply the ammount of solder you desire.
I also use bare wires, and I personally just use step 1 to remove the corrosion from the wires about once every 3-4 months. I hope this helps you out. Happy Listening.
Greg