Doubled Pawn: when and how?

Doubled Pawn is a weakness, but how do you exploit it?

I assume it's more evident in end game and clearly see that in such case,

But for exemple, here how do you exploit the weakness?

Or shall it be exploited in middle game, but how?

It's not always a weakness. In pawn endgames: As an attacking force it's a weakness because you can't create a passed pawn.

As a defensive force it's not a weakness.

In the middlegame it depends on a lot of factors. If it can be attacked - are they also isolated, does your opponent have a semi open file looking onto them, what pieces are still on the board etc.

All you have to do in the first example is 1.Ke3....Kf4 and everything goes. Black can never catch the pawn that runs. Study "Opposition" in endgames and it will tell you everything you need to know of these scenarios.

If they are on an half-open file, double rooks and just put alot of pressure on them. Then your opponent ties his pieces down to the defence, you create another weaknesses and you'll win.

I recommend Silmans: How to reassess your chess for exploiting doubled pawns in middlegames, and Aagards: Excelling at technical Chess for exploiting them in the endgame. 

Merci!