How can you skid / How to do skids / skidding

Thanks for all the replies.
Yep, I ended up getting a front brake instead of rear.
I don't know what the current cog is but, yep, I think I'll go and get a 48x19 fitted on the other side. Where would be the cheapest place to buy and fit it in London (central or east-ish), I wonder? ...

I think that the Fuji Track is a flip-flop hub (one side free, one side fixed), so you really don't want to put a fixed cog on the other side.