Its no different really to organising any other match mate.
Pick your charity, pick the venue and date, book it, then stick a thread up on here with all the details of the date, venue, draw times, costs for the day and what charity you re holding it for and sit back and wait for everyone to fill the slots[:T]
Its not difficult setting it up, but it can be a bit of a headache on the day getting everyone organised[

]
Be prepared for some to drop out at some point before the day, it happens all the time, nowt you can do about things that crop up[

]
In the few charity matches I have organised, Ive tried to keep things simple as far as the money side is concerned.
Simply divide the money in the pot (after taking out the peg fees) 50/50 between charity and prize money.
Payouts would be a simple case of section winners, not 1st 2nd and 3rd overall, as this gives everyone a chance of picking up a few quid for their troubles (you ll find that charity matches attract pleasure anglers too, not just hardened match anglers[

]).
So, for example.
Pegs available. 20
Peg fee 5 (x20=100)
Entry fee 20 all in x 20 = 400
After peg fees total left = 300
Split 50/50= 150 prize/150 charity.
You can divide the anglers into sections how you want to, either spread the money thin and have 4 sections of 5 or 2 sections of 10 to give a bit more to the winners.
I know it doesnt look like a massive amount for the charity, but you can easily top that up by asking everyone to bring along a little something as a raffle prize, and charge 1 a ticket/strip.
It might also be worth having a trip out to your local tackle shops to ask them nicely if they would be able to donate some sort of prize for the raffle, and then auction these off at the end of the day[

]
Hope that helps a bit mate.
BillG has organised some cracking days out, so he is well worth asking[:T]