In this context you could also use a numbered lists as simple and pragmatic way to describe activities, procedures or processes. The activity
diagram from tip-1-6 could then be documented as follows:

  1. Authentication
  2. Select a product
  3. Check customer type
    1. Private customer, add the VAT (value-added tax)
    2. Business customer, ask for the VAT-ID
  4. Create Invoice

In case you have to describe concurrent processes, activity diagrams see tip 1-6 are the better choice