InvoiceGeneratorSimplified.cs

public class InvoiceGenerator
{
    public Invoice GenerateInvoice(string watermarkText)
    {
        // ...
        // Invoicy things
        // ...
        if (!String.IsNullOrEmpty(watermarkText))
        {
            WriteWatermark(invoice, watermarkText);
        }

        // ...
        // More Invoicy things
        // ...
    }

    private void WriteWatermark(Invoice invoice, string watermarkText)
    {
        // ...
        // Watermarky things
        // ...
    }
}