Here are the guidelines that a number of participants in the MSA put together over a number of cycles.
I'm going to ask the moderators to make this a "Sticky", although this might be one of those things that the coordinator needs to be involved in.
For first-timers, the following summary of standard procedures should help (I am indebted to Ken Nadvornick for putting most of this together, and not objecting when I proposed to re-use [aka “borrow”] it):
(1) Take note of the MSA assignment, which you will find inserted into the title of the thread.
(2) Take your photo(s) according to the assignment guidelines. Try to shoot something new rather than grabbing from your archives.
(3) You can post your photo in the MSA Gallery or, you can host your photo elsewhere and link to it in the thread, or, you can post it as an attachment to a message in the thread and it will show as a clickable thumbnail.
(4) It's all for fun, there are no prizes, but at the end of the MSA, a previous winner will choose a “winner”, and the photographer associated with the "winner" will have the opportunity to choose a theme for the MSA in a couple of months time, and then choose the “winner” from the submissions.
Additional hints about uploading your photographs...
(5) How to upload if you are an APUG subscriber: As a subscriber you can upload to the APUG galleries. You perform the upload in the same way you would upload to the standard gallery, except you press the "'All subscriber galleries", you find a drop-down menu and press the Monthly Shooting Assignment, that’s it.
(6) How to upload if you are NOT an APUG subscriber: If you aren't a subscriber, you cannot upload to the APUG galleries. Instead, you'll need to attach your image to a post in this thread. Of course, it is a much better idea to subscribe instead.
(7) (For subscribers only) If you start out by uploading to the Monthly Shooting Assignment gallery, the system won't give you the chance to fill in the data fields that all the other galleries offer - i.e. to list the film and developer, location, and the other suggested information. If you like to list that information, just upload the photograph to a standard gallery, and then "edit" the upload. During the edit, you can change the gallery it is stored in - in this case to the Monthly Shooting Assignment gallery.
8) Most importantly, have fun!