Having major problems with wiring, need advice:

So I'm trying to help my sister wire in some new recessed lighting in her living room. I'm trying to run it off of an existing circuit box, shown below:



The bottom right switch goes to the porch light. The top right switch goes to outdoor security lights. The middle switch goes to the entry light, and the left switch went to a "mystery cable" that didn't seem to control anything.

Here's how it looked before when opened up:



My original plan was to pull the mystery switch, cap it, and use that slot for the new wiring. So I pulled out the existing box, installed an old work box, and ran new cable down from the attic, shown here still in the sheath:



However, when trying to connect my wires back together, I'm having a headache getting everything to work. I tried this, and only half of the switches worked - the bottom right (porch light) and the middle (entry light). At times, the top right switch would actually turn on the interior lights, which I thought I had wired to the left switch. Kind of a disaster.



Really not sure where to go from here, aside from calling an electrician. Any advice would be muuuuch appreciated.

The first thing that comes to mind is how did you end up with so many more wire nuts than when you started? Is there just one hot line coming into the box ( all switches on the same circuit)? If you didn't label the wires before you started, then you - or an electrician - will have to trace to see what goes where. You can probably do this yourself. Label as best as you can at this point. Maybe just a simple numbering system. Then start from scratch. Hook up one at a time, test to make sure you have the right result, then move on to the next.

First you need to put all the white back together in one big wire nut again. Hook everything back up the same as before so that you have only two wires left from the switch you removed.
Then take the white from you new cable and add it to the other whites.
Take the loose black that also connects to the other switches and connect to your new switch.
Cap the other black that is left over.
Connect the black from your new cable to the other terminal on the new switch.

Slownsteady is right. You have to bring your neutrals (white) together and provide power from your source (black) to the lights that need it.

Pictured below is a recent three-gang switch box relocation I did. Notice the minimal number of wire nuts ... even though I had a porch light, a chandelier (three way), and a switched receptacle. Notice the pieces of white tape that were the labels for each connection. This relocation went very smoothly and worked perfectly the first time.

You need to identify the main power leg (black) and then power each location accordingly. Your neutrals (white) can all be nutted together.