How to attack the Nimzo-indian double pawn ?

In the Nimzo-Indian (and some other related systems) black is giving up a bishop for a knight and doubles the white c-pawns in return. Nimzo game guides often state that this double pawn is becoming a target later on. 

But how exactly would one attack it ? What are the general strategies hiere and typical manoeuvers(apart from concrete variations) ? Should it be attacked with a knight ? Or blockaded and attacked in the endgame  ?  Or should one try to open the c-file and attack it with a rook ?

All of the above :)

Normal attack plans consist op. b6 -> Ba6 and Nc6 -> Na5 followed up with moves like Rc8 etc

1) However this is a SLOW plan and you cannot implement this in one go as a decent white player will blow you off the board and white has several ways to counter the plan. However in countering your plan he is not able to put his pieces in optimal positions allowing you equalize 

2) Also note that before you can succesfully attack the pawn on c4 you have to put your knight temporarly out of play on a5 meaning that white as a qualitative piece advantage. If white plays well your knight can be out of play for a long time and you will lose. 

White's  double c pawn in the Nimzo, also the French, is actually not a weakness, IMO.

c4 is often a really good move for white, and in these setups he often gets to play it twice.

Speaking as one that has played the Nimzo-Indian from both sides within my career, there are a number of things to keep in mind:

1) While Black is going after the weak pawn on c4, White is going after the King!

2) Blockading on the dark squares (with ...d6, ...c5, and ...e5) will often make c4 is "long term" weakness, so no rush to go after it.  Worry about your King first.

3) There is such a thing as winning the c-pawn too quickly!  If you go after the c-pawn with a load of pieces still on the board (or the c3-pawn for that matter, which may be even worse because it takes one less move for White to activate), then once you leave c4 to take care of other operations, White will be able to push the other pawn to c4, put his Bishop on b2, and open up the Dark-Squared Bishop, the one that you can no longer oppose.  With White having the Bishop pair, you don't want to open things up.

4) The "Pipe Dream" scenario for Black is to trade off the Light-Squared Bishops, a set of Knights, and all the heavy pieces, and get down to a "Good Knight vs Bad Bishop" scenario, and ONLY NOW to you actually proceed to go after and actually grab c4, which can't ever be guarded by a Dark-Squared Bishop.

Nice ideas, thanks !