(Photo annotations by Pyrojester, in case no-one speaks japanese. Original link above)
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As an alternative to JasonX's rather difficult approach to making the jack, I would suggest buying one for literally a few quid off somewhere like here
-Pyrojester
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Thick copper wire, Some PCB with an Etched circle and a hole in (In picture is the bottom) , and some sheet copper wrapped around a pen

... Becomes this, after some solder.

Check it fits before procceding, and that the middle pin doesn't touch the outer ring with any metallic contact.

Another glory shot

The end of a mechanised pencil (With a hole cut in the bottom) , a bit of biro tubing (the inside bit), More PCB (Again with a hole in the middle) , insulated copper wire, and a piece of metal that is cut to make contact with the outer ring of the charger.

The copper bit in the middle makes contact with the outer ring, and is covered with the pen lid so the copper is still exposed

The copper still makes contact through the bottom of the PCB

The Biro tube is inserted in the centre of the pencil cap, and glued in at the back

Red wire (+) is bent round the inside of the inside tube, Black one (-) is soldered to bottom of the board

Red wire is glued to the hole, so no slippage. Keep the wires seperate!

The Plug is attached, with the red wire (+) going to the central pin (the bottom of the plug) and the black wire (-) going to the top (the cylinder)

Check it fits into itself

Plug power adaptop into socket. Check for smoke. If anything smells, start again.

Pow! You're done