This is the "peach" ballot paper. The only thing I'd add is to remind people that you get a first choice and a second choice vote. If noone gets more than 50% first choice votes, second choice votes are counted. So, if Bullock is not a clear winner in the first round, low vote candidates get their votes redistributed to second choices.
I'm guessing Lib Dem DuWayne Brooks is the most likely to be in the second round with Bullock, but there is a chance the Tories or conceivably the Greens will. This means that if you want to vote for a left of Labour alternative, but can't stomach the possibility of a Con-Dem mayor coming first, put Bullock as your second choice. Conversely, if you can stomach voting Lib Dem and your priority is unseating Bullock, you can vote for a left alternative and put Brooks as your second choice. However, if you put Bullock as your first choice, your second vote is unlikely to count as there's not much chance Bullock won't be in the second round.
Further info: Here's the Londonist's helpful guide to voting in mayoral elections in London's boroughs. Here's an explanation of the complexities of the supplementary voting system.