Once you have defined a filter, you are ready to attach it to a resource. Each resource generated by Nova contains a filters method. To attach a filter to a resource, you should simply add it to the array of filters returned by this method: 
/**
 * Get the filters available for the resource.
 *
 * @param  \Laravel\Nova\Http\Requests\NovaRequest  $request
 * @return array
 */
public function filters(NovaRequest $request)
{
    return [
        new Filters\UserType,
    ];
}