How do I add a fixed cost or fixed income?

Go to Finance → Other and click + Fixed item. Fill in:

• Type — income or cost
• Description — e.g. "Adobe CC", "Office rent", "Recurring retainer X"
• Monthly amount — what it brings in or costs each month
• Start date (optional) — from which month the item counts

After saving, the item is automatically counted in all subsequent months in the profit calculation.