Tin Wires and Switch
Use a pair of helping third hands to secure wires/switch in place. Appy small amount of solder to the tips of the wires and the terminals on the slide switch.
Connect Wires to Switch
Solder both wires to two terminals on the slide switch (one in the center, and another next to that). Cut a piece of shrink tubing and slide that onto the wire to cover it up. Apply some heat to secure the connections.