Welcome to the habit of AAR writing. Hopefully you'll find it as addictive as all the rest of us.
Don't worry too much though about the style, do whatever you find easiest to begin with. Later on you can experiment if you want to.
However a few tips that might help.
1. Decide how present you want the game mechanics to be. I, for example in my AAR (that is also my first) do have notes that explain the game events, and when I change the DP sliders. Some AARs include much more detailed in game information - look at Peter Ebbesen's 'Persian Tales' for the info reports at the end of his story-posts as an example. Or you could make direct mention to game mechanics at all, but just imply them through your description of events.
2. Play ahead for a few years. This gives you some idea then of where the AAR is heading when you write your posts, which is a useful aid.
3. Download Jos Theelan's EUReader. This is a very useful tool that tells you various things about your nation in comparison to others. Also it sorts out the history log into categories which is priceless at times.
Here is a post which he participated in. The link is in his sig.
4. Don't be afraid to change styles half way through.
5. Have fun. This is the most important.
And I'm sure we'll all have fun reading about it.
PS - Sorry to sound a little like I'm lecturing, do it however you want but I have found all of the above to be useful.
Edit: added link