I've never done one either, but I would imagine the logical steps would be to contact the school and seeing if they'll allow you to use their hall for the reunion. And then put ads in all your local newspapers advertising the fact.
Are you a member of Friends Reunited? If so, put an ad in there too. If you're not, might be a good idea to join.
Yes, don't
At the only one I went to the bullies were still bullies, the cliques were still in place and the rest of them were trying to score points off each other.
The one good friend I went with and I sat in a corner, had a damn good laugh at the lot of them and left early to go for a fish and chip supper.
Hope you went to a friendlier school than I did!