Connect the source wire to the bottom terminal of the switch and the other black wire to the top terminal. This video shows you the process in detail for connecting 2 electrical wires together.
Belkin Official Support Wiring Your Wemo Wi Fi Smart Light
How to connect wire to switch. Simply strip the wire to the length specified and push the wire into the hole. Pull the wires apart. The new line is attached to a three way switch in the original box where there is a red and white wire to the switch and black to the white from the 2 wire line from the appropriate light source. The traveler wires dont need to be labeled because either traveler wire can connect to either traveler screw terminal on the switch. Connect the black wires to the switch terminals. Connect each of the other two wires to one of the light colored traveler terminals.
To wire the new switch connect the labeled common wire to the com terminal usually bronze or dark colored on the switch. There is the existing 3 wire cable to one switch. All that you have to do is connect the like colored wires together with wire connectors. It is very important to connect the ground wire to the switch as well. Three 2 wire lines from a light which may be the power source. In this picture you can see the strip gauge on the back of the switch.
Trim 12 inch of insulation from the ends of the black and white cables. The white wires are wire nutted together so they can continue the circuit. This gauge is when you want to use the quick connect method when wiring a light switch. Now as for the ground wire. The green screw on the 2 way switch is for the ground so all ground wires should be connected as seen below. The switch simply interrupts the current so it.