The Autumn--or Fall-theme is one most people will have been taking shots for in the last couple of months. Usually, Submissions stay open for a week, but can fill up sooner, especially when people have been anticipating the theme.
In order to finish the contest before Christmas, it would be a good idea to get Submissions open soon.
Thanks for hosting! The easiest way to do it for the first time is to use a previous contest as a template. CaliforniaPeggy hosted the last seasonal (September) contest --for Summer- so if you follow what she did, you'll do fine. Copy and paste what you can, and then change the variables, like the theme name, and insert the appropriate links to the threads that you will make for this contest. Once you determine the pattern for constructing the threads, it will then just be a matter of filling in the blanks.
Link to September Submissions https://www.democraticunderground.com/1036135336
When you get ready to post the Preliminary threads, you can do the same thing. For example, here's the link to announcing the Preliminary threads of the Summer contest --which posts in Photography--but you'll notice the Preliminary Threads (and Finals) post in GD for everyone to vote.
https://www.democraticunderground.com/1036135875
And so on, for the Finals thread, announce in Photography, but post in GD for voting, and then the Winners announcement the same process. For each of them you can go back and see how California Peggy set them up in September.