To cash flow your budget, open the Budgeting module (1) and go the budget (2), category (3) and account (4) you want to adjust in cash flow.
Now click the small blue pen symbol next to Cashflow in the macro area (5).
Select milestone-based (1) and choose your first milestone.
For example, you can schedule the camera rental payment in a 50% (2) installment seven days before (4) shooting begins (3), and add a second milestone by clicking "+" (5)…
…to add the final payment of another 50% seven days after shooting has ended. (1)
Click Ok (2) to proceed…
…and see that a green circle (1) indicates this account has cash flow data, whereas the details can be seen in macro panel (2).