The small junction boxes are simple - especially if you're just connecting 2 MLs. With 3 or more, it can get a little tight, and I'd recommend using the larger box (which is really just the smaller box oriented perpendicular to the wires in a larger enclosure). Definitely consider the larger box for 4, but 3 isn't that bad.
Carefully cut off about 2 inches of the ML cable's outer insulation (cut off slightly more than the length of the junction box) and unwrap the 3 bundles of wires from their foil. Be careful not to disturb the insulation on the individual wires. Use the included tool push to the wires into their appropriate slots (follow the color coding). Make sure you push the wires down firmly so the slots cut through the insulation. The first set of wires should go all the way to the bottom of the slot. Start with the 3 uninsulated grounding wires, then work your way down the junction box. Try to keep your wires organized so the don't get twisted around too much. Trim the excess wire. Repeat.
My wife teases me because she says I'm not very handy (actually, I'm just lazy), but I've done this 6 or 7 times (I've linked my BS9000, BeoPort, BL3500 and BL-Active, twice because I moved) and my connections have all worked fine.
Stan