Turn off the power to the cable, at the main panel.
Locate the knockouts on your junction box, the precut holes that exist to allow cable to enter the box. With a hammer and screwdriver, open a knockout for each cable that will be entering the box.
Connect the cable, with cable clamps, to the outside of the box. Push the cable through a knockout, into the box, and thread a locknut over the cable, onto the threads of the cable clamp. Finger-tighten the locknut, then tighten it fully with the screwdriver. Connect the white wires of the cable to the white wires within the junction box, and the black hot wires together. Connect the bare copper wires from the cable to the ground wire in the back of the cable box.
Plug in the soldering gun and allow it to warm up. Touch the solder lightly to the tip of the soldering gun, allowing it to melt.
Drip solder all over your exposed wire connections, until it thoroughly covers them.
Wrap the exposed connections in electrical tape. Replace the junction box cover. Turn the power back on.