I know this is easy stuff compared to most of the 'How To's on here, but I'm not mechanically inclined and looked for a good guide and couldn't find one. Hopefully this will help somebody else, as impractical as me!
The parts supplied by Rizoma.
Not all the parts are needed, but you will need bar ends (in this case Rizoma 23mm).
Start by removing the old mirror, just undo the marked bolt. Note that the right mirror (on the throttle side) is reverse threaded so is removed by turning clockwise.
There are several ways to tidy up the gaping hole you leave behind. I elected to just turn the clamp upside down, as I couldn't find plastic hole caps that would fit (1/4" I believe).
And then remove the stock bar end. I used a combination of increasingly wide screwdriver blades and a pair of pliers to gently prise it out.