The way I would like to see it is like this:

1. First it should have 2 modes: auto and manual. Auto would generate all columns equal based on single column and gutter values. Manual would let you set your columns and gutters at whatever values one wishes.
2. It needs to be draggable with the mouse and you should be able to see near the grid line the values of the column you are changing.
3. After the grid is made mouse interactive, the way you would create one grid could be something like this: First you set the first line mirrored by the last witch would set the total width of the grid and the margins and then on the next step you drag to set the columns and gutter values. Visual cues about their values are needed.