How to make a basic construction system?

I want to make a basic blocks-based construction system like terraria.

So,

I guess I've to use Drag n' Drop function,

but I don't know where to start.

This is what I want to do,

my world is block-based, so imagine every [X] is a 32x32 block.

Then the world would be something like this:

[X][X][X][X][X][X][X][X][X][X][X][X][X][X][X]

And I want to put a new block (for example wood block) in any place of the map, but I want the block to match with the [X] blocks.

So it looks like this:

[W][W]

[X][X][X][X][X][X][X][X]

[W] = Wood Block

So, there the wood block matches the rest of the world blocks,

this would be an example of what I don't want:

        [W]       [W]

[W]   [W] [W]

[X][X][X][X][X][X][X][X]

So how can I make a simple system to put block in the map,

and those blocks to match the blocks on the ground?