Put the dimensions of your osu! client here!
For this next part, open up your skin's skin.ini file and look for the ColumnWidth for your desired key mode.
Use this method if your skin has the same width for each column.
Use this method if your skin has different widths for each column. Simply add up all of the widths and put the sum in the sum textbox.
Replace the ColumnStart value in your skin.ini to the one shown here!