For tips on Soldering, see How to Solder and Desolder Connections.

Attach the disconnected wire to its joint using solder and iron tip. Place the tip of the soldering iron against the solder pad.

Melt enough solder onto the solder pad.

Remove both the solder and the soldering iron tip from the connection as soon as enough solder melts onto the pad.

Don't forget to use gloves.