Oh well done
Do it well, look on it as good experience, see that those paying you are pleased, and perhaps ask afterwards if you might use them as a reference. Might well be worth asking if there are other options for you to work for them again.
On the practical aspects of it re money, etc., have they offered you a contract ? one that clearly lays out their expectations and yours, the parameters/scope of the report, how it is to be presented, and how any issues will be resolved. Mind a week is a week, not a never-ending 'can you do this too? ', and dragging on and on.
Make sure that you have things clearly set up so that you can be paid using whatever system they use. Usually in short contracts it's by cheque, but Bank Transfer is becoming more common.
I know two people who only bank with 'ethical' banks (the co-op for instance) and they have huge issues being paid.
They will (or they ought) to ask for you N.I. details, usually they set you up as a 'contractor' for stuff like this.
It's rare to be paid as soon as you're finished. Anywhere between one and three months is normal.
I hope it goes very well
I hope it's fun to do, that it leads to more work that you enjoy too.
atb,
M