If you don't have at least a cheap multimeter, I suggest you get one. All you need is something to help you determine what connects to what. Use it in conjunction with what fonebone has said. The tip is right, the gold ring underneath is left, and the massive gold ring is ground. Look up a TRS connector if all else fails.
Now, just to be sure, set your multimeter to check continuity (Ohm symbol).
If you touch one of the rings and one of the prongs you solder the wire to, and you get 0 ohms (or something very low) then that prong belongs to that ring. If it's the right ring, then solder your right wire(s) to it. Same with the left, and same with ground. I don't know what Null wires are like, so I can't tell you how many of what there are. Most of my work is with Canare StarQuad, so I have a sheath which is ground, and two wires for left, two for right. Or if I'm making a guitar patch cord, all 4 are the same channel.
EDIT: You should be safe following the above diagram. Personally I don't work much with 3.5mm connectors, just 6.3mm, so it's a fair bit easier with them.
