An initial polar field of +-10 Gauss was placed within 20 degrees of the poles, and a BMR was placed in each hemisphere, such that the trailing spot has magnetic field of a polarity opposite to the polar magnetic field in that hemisphere. This automatically set the leading spots of the BMR to be opposite in polarity of magnetic flux.
Such BMRs were placed at regular intervals till the polar magnetic field was totally cancelled, and then became opposite.
The following figure is a butterfly diagram of the simulation. There is nothing like a butterfly in it, it is named like that for a different reason.
On the X-axis is the no. of time steps,
on the Y-axis is the latitude of the surface of the Sun,
and the colors indicate the value of magnetic field on a particular latitude at a particular time(averaged over all longitudes).