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This is done in the tutorial, so if you did not do it originally, you will need to start a new safe slot for it. Luckily it is very quick.

You will need to progress the tutorial into the room where you first get a bomb. Do not use it like the tutorial tells you though, but take the bomb back into the first room and drop it in the center of the upper wall.
It will open a secret passage. Pass through it and you will get the achievement