1 - Simply enter the required levels in the top entry box and click OK.
2 - Fill in the From and To fields, the number of levels required, and click OK.
3 - Fill in the From and To fields, the interval required, and click OK.
Note that in cases 2 and 3, if either or both of the From and To fields are left as the default values (the data min and max), then contours will not be plotted at these levels, but will be fitted in between.
Note also that contours accumulate unless explicitly reset.