public static void CreateSingleCategoryScheduleWithGroupedColumnHeaders(Document doc)
{
    using (Transaction t = new Transaction(doc, "Create single-category with grouped column headers"))
    {
        // Build the schedule
        t.Start();
        ViewSchedule vs = ViewSchedule.CreateSchedule(doc, new ElementId(BuiltInCategory.OST_Windows));

        AddRegularFieldToSchedule(vs, new ElementId(BuiltInParameter.WINDOW_HEIGHT));
        AddRegularFieldToSchedule(vs, new ElementId(BuiltInParameter.WINDOW_WIDTH));
        AddRegularFieldToSchedule(vs, new ElementId(BuiltInParameter.ALL_MODEL_MARK));
        AddRegularFieldToSchedule(vs, new ElementId(BuiltInParameter.ALL_MODEL_COST));

        doc.Regenerate();

        // Group the headers in the body section using ViewSchedule methods
        vs.GroupHeaders(0, 0, 0, 1, "Size");
        vs.GroupHeaders(0, 2, 0, 3, "Other");
        vs.GroupHeaders(0, 0, 0, 3, "All");

        t.Commit();
    }
}

/// <summary>
/// Adds a single parameter field to the schedule
/// </summary>
public static void AddRegularFieldToSchedule(ViewSchedule schedule, ElementId paramId)
{
    ScheduleDefinition definition = schedule.Definition;

    // Find a matching SchedulableField
    SchedulableField schedulableField =
        definition.GetSchedulableFields().FirstOrDefault<SchedulableField>(sf => sf.ParameterId == paramId);

    if (schedulableField != null)
    {
        // Add the found field
        definition.AddField(schedulableField);
    }
}

Public Shared Sub CreateSingleCategoryScheduleWithGroupedColumnHeaders(doc As Document)
    Using t As New Transaction(doc, "Create single-category with grouped column headers")
        ' Build the schedule
        t.Start()
        Dim vs As ViewSchedule = ViewSchedule.CreateSchedule(doc, New ElementId(BuiltInCategory.OST_Windows))

        AddRegularFieldToSchedule(vs, New ElementId(BuiltInParameter.WINDOW_HEIGHT))
        AddRegularFieldToSchedule(vs, New ElementId(BuiltInParameter.WINDOW_WIDTH))
        AddRegularFieldToSchedule(vs, New ElementId(BuiltInParameter.ALL_MODEL_MARK))
        AddRegularFieldToSchedule(vs, New ElementId(BuiltInParameter.ALL_MODEL_COST))

        doc.Regenerate()

        ' Group the headers in the body section using ViewSchedule methods
        vs.GroupHeaders(0, 0, 0, 1, "Size")
        vs.GroupHeaders(0, 2, 0, 3, "Other")
        vs.GroupHeaders(0, 0, 0, 3, "All")

        t.Commit()
    End Using
End Sub

' <summary>
' Adds a single parameter field to the schedule
' </summary>
Public Shared Sub AddRegularFieldToSchedule(schedule As ViewSchedule, paramId As ElementId)
    Dim definition As ScheduleDefinition = schedule.Definition

    ' Find a matching SchedulableField
    Dim schedulableField As SchedulableField = definition.GetSchedulableFields().FirstOrDefault(Function(sf) sf.ParameterId = paramId)

    If schedulableField IsNot Nothing Then
        ' Add the found field
        definition.AddField(schedulableField)
    End If
End Sub