To solder the wires to the relay, tin the end of the required wire, and bend the tinned end of wire in the middle with needle nose pliers to form a "U" shape. Slide the loop through the hole in the relay connection, and crimp it tight with the pliers for a strong physical connection.
Clean the tip of your soldering iron (wipe it on a damp sponge or with a paper towel), and apply solder to the connection.
If you have trouble getting the solder to flow onto the relay's terminals, polish them with fine sand paper just before attaching the wire for soldering.
The solder should flow evenly between the relay terminal and wire with no blobs or spikes. Be careful not to use too much solder. In a perfect joint you should still be able to distinguish the individual strands of wire.
Don't overheat the relay. The soldering process should only take 5 or 10 seconds. If it takes longer, remove the iron, let the connection cool completely, re-clean the soldering iron tip and try again (only use the shiny portion of the soldering iron tip.)
Excess solder can be removed by wiping off the tip, holding it to the connection allowing gravity to draw the solder down to the tip, removing the tip from the connection, and wiping it again.