QuickTourBuilder.cs

public class QuickTourBuilder<T>
        where T : BaseQuickTourPage, IQuickTourLauncher, new()
  {
      public static QuickTourBuilder<T> Initialize()
          => new QuickTourBuilder<T>(new T());

      public QuickTourBuilder<T> Next(BaseQuickTourPage page)
      {
          _pages.Last().NextPage = page;

          AddPage(page);

          return this;
      }

      public IQuickTourLauncher Build()
       => _pages.Select(page =>
                       {
                           page.TotalSteps = _pages.Count;
                           return page;
                       })
                .ToList()
                .OfType<IQuickTourLauncher>()
                .First();

      private QuickTourBuilder(T initialPage) => AddPage(initialPage);

      private void AddPage(BaseQuickTourPage page)
      {
          _pages ??= new List<BaseQuickTourPage>();

          _pages.Add(page);

          page.ActualStep = _pages.Count;
      }

      private IList<BaseQuickTourPage> _pages;
  }