Arc Warden is one of the most versatile heroes in Dota. His base kit has the typical stuff like nukes and some sort of protection but what makes him unique is that he can create an exact copy of himself. This copy can do everything what the main hero can do, including casting abilities and using items. Because of this his power is basically doubled on ev erything he buys.
Playing Arc Warden requires quite an amount of micromanagement. Playing one hero is tricky, playing two at the same time can be really hard. Luckily there are some nice control settings, that make it easier to play with Arc Warden.
On the left side of the screen you can see icons for both your main hero and your copy. You can click your Double even if it is not summoned and see what his cooldowns are doing. This is useful if you want to check if your double has tp or Hand of Midas ready.
When your double is active you can easily see that way how much HP you or your double have and one icon starts blinking when the affected hero is attacked.
Players should set their keys that way, that they can easily switch between Arc and his Double, this differs from person to person because everyone is comfortable with somethign else. For me I have set my main hero on "1" and my double on "2" and I can switch between the two with the "tab" key.

As Arc Warden you have two roles, whenever you are not in a fight you should send out your double to push out lanes, preferably those you don't want to be at. That way you can savely push out diffcult lanes while farming with your main hero insafer areas like your side of the jungle. If you are up to it you can push two waves at the same time, one with your double, one with yourself. You have to be sure you can have an eye on both characters so you don't end up dying or feeding your double.
When you have your core items and your team is ready for fights, you should use your double to deal everything twice or, if you can manage it, split up all your abilities on two different targets. The key is to switch quickly between you and your double and cast all your abilites and all your spells when they are ready. Sin ce Arc Warden has very low cooldowns on all his abilities, his ultimate included, you will do this many times. It sounds hard at first but if you do this often you will get better at it and used to it.
When I started out as Arc Warden I was really bad with him now I have a 55% winrate on him.

What I noticed is, a lot of people don't know how to react to Arc Warden. Many players see Invoker, who is considered to be evenly hard (or even harder) than ARc Warden and because of this they are used to him and know how to play around him. How many times do you see people pick Arc? A lot less. And because of this people are confused how they shoudl deal with him. This makes him in my opionion a good choice to pick in public matchmaking (but less desirable in team matchmaking, where people are better coordinated). As long as you push out with your double and keep your hero relatively safe, you should be good to go and eventually your enemies will either tilt and try to get you, which results in them doing a lot of mistakes or you will have an easier match and just finish one tower after another until you get their ancient.

You might notice that I didn't went the usual route for my guides in this one. That is because I believe that Arc Warden has so many ways to play, you can hardly cover it with s normal guide. Playing Arc Warden means to play creative and that's what I want to encourage people here, I give examples of general good items on him and how to use them but no strict restrictions what to choose.