A few of my friends just got aoe3 DE and I realized after playing our first match vs bots that this game actually has a good amount of complexity to a new player. I noticed this when I saw them using their vils to fight treasure guardians and building mills in age 1. So what are some golden rules you would tell a friend that's new to the game? They're interested in playing pvp as soon as possible so I just don't want them to be discouraged when they inevitably get steamrolled while they're still in discovery.
It might be best to just let them mess around a bit with the campaign and single player matches first so they can explore the game and it doesn't feel like homework.
When they're ready and motivated to focus on multiplayer, ask them to pick one civ to focus on first. Best might be one with some sort of "training wheels" mechanic like Ottoman villagers, but as long as they avoid the really complex ones like Mexico it's probably fine.
Let them learn a standard age 1 build for that civ and to have some strategic plan for what happens after that. Macro and micro will come with practice. If they really want they can play multiplayer with that, although it might be more enjoyable to do some more single-player until they can beat the expert AI every time with their chosen civ.
Starting out with Art of War seems like a good idea to get the basic ideas and feel of gameplay. If they have played other RTSs, they can quickly get the gist through it. And then picking up a simple civ and learning its BO should be good.
Make sure not to introduce too many new concepts all at once. Don't nitpick about herding or build order, for example. They don't need to know that you should research Steel Traps as soon as you hit age 2 unless they're quite advanced RTS players.
The Historical Battles can be played co-op now. That may be a good environment to learn some of the basics of the game without the pressure of a multiplayer match or the feel of grinding vs the AI all the time.
my advice would be watching people slightly better than them play, and then try to mimick it and go from there. Keep it simple, if you noticed in a previous recording you built a mill whilst theres hunts somewhat nearby fix it. If you noticed you had too much of one resource adjust vills accordingly. Stuff like that, I wouldnt focus on build orders at all unless its a very simple one that they can practice over and over to get basic macro down and then expand from there.
You can go on Youtube and search "aoe3 basics". There are some videos that explain the very basics for a beginner. Then, after they get the basics down right, they can move on to searching more advanced topics like "aoe3 how to herd".