If you are planning to do the same first look at where you want to put the socket. Think about the length of cable from the plug to the device to help you decide where. I chose to put mine under the seat which means I do not have to wire it into the ignition circuit or put a switch in to stop anyone draining my battery.
The 12V Waterproof Socket Cigarette Lighter (C) came from Amazon and was the most expensive part at £4.50. Both 10A Blade Fuse (A) £0.52 and 30A Blade Fuseholder (B) £2.29 came from Maplin which was bought in store.
Firstly strip, twist and solder the joins. One the red cable I reused the connector which came with the 12V socket hence the extra join. You will need another.
A mate let me have some spare black wire and also provided tubing which covered the joins which was then heat shrunk using a heat gun.
I decided to use black insulation tape over the top too to make sure that it was water tight.
I would recommend getting a multimeter and checking the resistance just to make sue all the joints are good and there are no short-outs. Do not install the fuse yet.
Attach the socket under the seat to a cleaned area using Velcro. This means you can move it out of the way to make more room under the seat for luggage. When positioning make sure you leave enough room to be able to plug & unplug and that the plug can sit in the socket cleanly when the seat is put back into place.
this new wiring first connect the red positive (+) cable to the battery. I actually used latex gloves to avoid accidentally shorting out the battery or the motorcycle wiring. Clean up the current connections to the battery. I put a tiny smear of Vaseline on the connectors to ensure of a good connection.
There was enough gap to push the connector through the back of the red protective terminal cover.
I did not wear latex gloves for this bit as the bike is negatively earthed so this cable is normally okay if it touches any part of the bike.
Finally put the fuse in.
You can now put the 10A (amp) fuse in. I tested it by plugging in just my SatNAV cable which had an LED which lights even when the SatNAV is not connected.
Providing your motorcycle is charging the battery correctly it should be possible to have a mobile charging without the engine running. Anything like a pump make sure the engine is running or you might be push starting your bike next time. Basically if in doubt run the engine.
The other advantages of having this not connected as part of the ignition circuit are you can lock a mobile on charge under the seat (make sure no one sees you doing this) while you go get some lunch and do not have to leave your keys in the ignition.
The main disadvantage is that if you have soft panniers which lay across the seat then it is difficult to get under the seat. As they say think about Location Location Location.... and what and how you are going to use it.