<font color=black>Basically set the block's X Position to the XMouse (X Position of the Mouse) devided by 32, convert that from a float to an int (Basically round it down), then times it by 32 again. (Replace 32 with the grid size you want), It's really that easy! Believe me!
So basically even in code:
int gridSize = 32;
block.Position.X = (int)Math.Floor(Cursor.X / gridSize) * gridSize;
In Construct2 it should be farely similar:
(Declare a integer variable called 'gridSize')
block | Set X position to ( floor(XMouse/gridSize)*gridSize )
I haven't done C2 in a while but it should be something similar to that, if you still need an example please let me know!</font>