Strip the ends of the wires.
Use fine sand paper, emery cloth or steel wool to clean any corrosion or coating off the wires.
If using shrink tube to cover the connection slide it down one of the wires now.
Twist the wires together making a good mechanical connection.
When hot, wipe the soldering iron/gun tip clean with damp sponge or rag.
Apply a very very small amount of solder to the tip of the iron/gun, just enough so it has a shinny coating. This is called tinning.
Heat the joint.
When hot apply rosin core solder to the opposite side.
As soon as the solder flows filling the joint remove the heat and solder.
Try not to move the wires till the solder has hardened. Usually a couple of seconds.
You should have a nice bright shinny joint and good electrical connection.
If using shrink tube slide it over the joint and heat.
If the joint is going to be exposed (out in the open uncovered), you can use steel wool or solvent to clean of the excess rosin. Then put on a light coat of clear lacquer. This will prevent oxidation and keep it nice and shiny.
If you are soldering a number of wires, when the iron/gun tip gets dirty or has burned rosin on it, clean and re-tin the tip.