Go to a Deal on HubSpot and navigate to the section on the right panel that says “HubSpot e-conomic”:

Click on “Create invoice/quote/order”

Once the window pops up, you will get multiple options to manage your invoice/quote/order manually.
Add relevant invoice details on this window or click on the blue colored '+' icon to create a new invoice/quote/order:

Select customer, currency, layout and account details to get started:

Select invoice specific details such as Invoice date, preference, payment terms and delivery date

Add new products to your draft invoice by selecting the "+ Add Line Item" button.
Important: Please note that you can select products using this option only if they are already present in e-conomic.

Split your invoice into two by specifying the percentage ratio. Click on the three-dots and select "Split"

Duplicate your invoice or quote by clicking on ‘Duplicate’. Click on the three-dots and select "Duplicate"

If you need to come back later and update the details, click on "Save Progress"

Once you are ready to move this draft invoice to e-conomic, click on "Sync"

Close the window and view your invoice in the "Attachment"/"Notes" section within the HubSpot deal.