Alright, so.
Experience is the best thing for you at this point; nation roleplays have a certain style to them that isn't found in the typical roleplay. Identifying it and knowing how to roleplay in a nation format is a good start.
When it comes to GM'ing in general, let alone nation, ya need to keep folks organized and promote worldbuilding for the said nation. Half-assed nations won't get too far, at least in my opinion. Have a system as to knowing what is going on where; I like to do it in this format:
Travincal; Senator's palace
(insert a scene with some important senator musing how to betray his ally while describing the setting if used for the first time)
Western Front; South of Balthanoli
(insert scene about the general preparing to go to war)
Something with a format so it isn't all that confusing.
Have systems. How battles will be resolved, stats or not (both can work just fine), army and technology limitations if applicable, etc. If there's a chance of something popping up, have a plan for it, such as if someone decides to make submarines in a medieval setting and go with the excuse that there is small air pipe coming above water or "medieval bombers". That probably won't be an issue here, but I have seen folks try to do that before. Set the tech limits. If it's medieval, specify if guns are allowed or not.
A co-gm that knows his/her stuff would be good, especially if it's your first time.
Checking with folks that have GM'ed in nation roleplays before could be of benefit when it comes to advice.
Besides that, pay attention to what others are doing and learn from them. If it's a system that works, use it. Don't blatently copy content, but if there's an ingenious idea on how to keep track of IC posts, then that sort of thing should be more common and in use since there are folks that would appreciate such a system (bringing it up to the said GM before using the idea would be respectful, though).
Random collection of thoughts, so take what you will.