1) Pick a date
2) Pick a venue
3) Advertise/send out invites with details of the charity, cost, prizes, format for the day etc
Charity days are often team events such as best 2 from 4 stableford format. If you start asking people which date/venue is best for them you will get loads of differing answers so you will probably find it easiest to pick a suitable date and location then people will commit or not.
If you want everybody in at roughly the same time and all present for the prize giving I would go for a shotgun start.
Format can be Texas Scramble, 4BBB, Bowmaker.
Our most successful event this year (not a charitable one but could have been ) was a team event and all prizes were for the team rather than individuals including nearest the line, nearest the pin ands nearest the peg.
The golf society I play in has a charity day in the summer, due to the large numbers that attend it's vital they use a course that support a two tee start, just a normal Stableford comp with awards for longest drive and closes to the pin. When they tried it with a single tee start the last group came in a couple of hours after the first one which was a problem as far as catering. As someone else suggested a shotgun start would also help getting the finish time as close as possible for all groups.