1 - To enable Milestone-based invoicing for a project, navigate to the Project's settings and activate the Milestones option.

2 -  Once Milestones are enabled, create a project intended for sale as a service with invoicing based on Milestones.

3 -  After creating the project, set its price and generate a sales order for the total amount. Note that a sales order is distinct from an invoice. Confirm the sale order to proceed.

4 - Upon confirmation, three new smart buttons (Project, Milestones, Tasks) appear. To configure the project's phases, milestones, or objectives for sale, select Milestones.

5 -  In the Milestones menu, customize each Milestone or create new ones. Specify the Quantity % to invoice upon reaching an objective.

6 - Now, to invoice a completed phase, access the project's dropdown menu, click on project updates, select the desired Milestone, mark it as reached, and navigate to the sale order (via the smart button). Save and close the sale order.

7 - Following the previous steps, view the sale order displaying the total amount and a Delivery column indicating the accomplished Quantity %. Invoice the client by selecting "Create Invoice."

8 - The resulting invoice reflects the total amount based on the reached Quantity %. Proceed with the standard confirmation of the invoice and register a payment.

Later on you just have to repeat from step 6 to 8 so that you invoice the sale’s order total amount. And don’t worry about an increase in the Delivery Column while repeating this process, you will only invoice what you customized in your Milestones.