By about 12 Sep 2016 you will see a notice like 2014 Community Moderator Election (from the last election) appear to announce the 2016 Community Moderator Election for GIS SE.
Moderator elections occur on an as needed basis, typically each year or two. In this case the moderation team seeks to expand its number by two, in order to meet the growing demands of the site, particularly during working hours of the Asia Pacific time zones where I am currently the only one living.
I would encourage all users eligible to nominate, to contemplate whether they might wish to do so now, so that when the Nomination Phase opens on about 12 Sep 2016, you will be in a position to quickly post a small introduction to yourself and why you think you would make a good moderator.
For generic information on the Stack Exchange election process I recommend reviewing There's an election going on. What's happening and how does it work?
To see how past elections have been run for GIS SE our Meta Q&As tagged election are probably the place to start.
Feel free to ask questions about the upcoming election any time from now, but there is nothing formal that potential nominees need to do until the formal announcement of the 2016 Community Moderator Election.
Note that Community Moderator Elections are run totally by Stack Exchange staff and not community moderators. The GIS SE moderators simply say to them that they think it is time for an election for X new moderators (in our case two) because ..., and if they agree (which they did instantly), then they handle the process. I asked permission from them to post this advance notice because for the last election we only had one nominee for most of the nomination phase.
The 2014 election had 1,704 eligible voters, this one will have at least 2,932.