You’d get the coordinates under the mouse cursor to get the location, then you can just round the coordinates to the nearest grid size (i.e if you grid is 100 units per cell, round to the nearest 100).
It is genuinely that easy 
You’d get the coordinates under the mouse cursor to get the location, then you can just round the coordinates to the nearest grid size (i.e if you grid is 100 units per cell, round to the nearest 100).
It is genuinely that easy