Obviously if you are already completely au fait with the operation of your 3 ballot papers then I am being U, and a bit patronising, in which case, sorry, move along, nothing of interest to see here.
But my impression is that the vote for GLA members has been comprehensively overshadowed by the mayoral vote, so many people may have forgotten its details.
Paper 1 (pink) is your mayoral vote - only point to remind you of is that you can safely vote for whoever you fancy with preference 1, safe in the knowledge that you have your second preference to use as your "Keep Out Boris/Ken" fallback.
Paper 2 (yellow) is your vote for your local GLA member, standard first past the post voting system.
Paper 3 (orange) is the interesting one. You vote for a party, not an individual (except for the "independent" candidates), and the proportion of votes cast on this ballot governs the allocation of "top up" seats from the party list into the GLA. Hence, a party that has a steady level of 10% support in every area, but not enough in any one area to win a local seat on FPTP will get recognition via this ballot and end up with 10% of the seats. (it's complex in practice, but that's the gist, see www.londonelects.org.uk for full details)
There is a threshold of 5% below which you will not be allocated any seats. Therefore, if you do not want to see the BNP represented on the GLA, it is in your interests to go out tomorrow and use your third ballot for any other party you can stomach. We want to dilute the BNP vote to less than 5%.
The National Front are also on the ballot, but their chances of polling more than 5% are negligible. There is also a reasonably professional party rejoicing in the charming name of "Christian People's Alliance - Supporting Traditional Marriage", I don't know if they have any chance of making the 5% target though.
At the last election one BNP candidate did get a seat by the top-up method - fringe politics being what it is, he is now an independent.
So in summary: do whatever you fancy, but if you do not want the BNP represented on the GLA I would recommend you find another party you prefer and use your third ballot tomorrow.