Finding WEAK squares. How do I practice/tactics ?

I have no concept of scanning for weak squares.  

Can someone point me in the right direction o getting started?

I like the answer in #3.

There are at least a couple of scenarios where I think it's still important, even at lower levels though.

If you advance pawns around your king so that they are protecting a single colour, then it's often important to keep the bishop of that colour for defence. Otherwise they can be key squares for your opponents attacking pieces.

Second is when a pawn is backwards i.e. behind the adjacent pawns and can't be safely advanced. If you have or can create an open file against this you can put serious pressure on the pawn. Even if you can't win it you may be able to tie up your opponents pawns in defence.

Take the following position. Which square in it is weak?

The weak square in that position is f6. White can make use of that by playing Ne4 and Bg5. White could plant a piece on f6, and since it will be difficult to dislodge it from there, Black will have a hard time getting his pieces out and his king won't be very safe.

You might say: "Isn't f3 also weak? Black does not have a lot of control over that square either." In general, a weak square is a square that is found in a player's camp but is controlled by his/her opponent. The opponent can make use of it by occupying it with a piece or a pawn, thus cramping the other player's position. Take the following situation from a game between Karpov and Kasparov:

White is paralysed here. His minor pieces are not doing anything, his queen is a spectator and Black's knight on d3 stops the white rooks from getting into play. Kasparov made use of the weakness of the d3-square and ended up winning that game.

Now, take this position from the game Caruana–Carlsen, Tromsø 2014:

Though White's d6-knight may look menacing, Black's pieces are well coordinated and White has a weak pawn structure. Carlsen won the game.