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

Click on “Create invoice/quote”

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

Select contact, currency, tax and account details to get started:

Select invoice specific details such as Invoice date, Due date, Status and Tax preferences

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 Xero.

You can also add custom preferences to products by clicking on “Customize Accounts, Taxes and Amounts”:

Split your invoice into two by specifying the percentage ratio. Click on the three-dots and select "Split"
IMPORTANT: You can only split an invoice as long as it is in the 'DRAFT' stage. Once an invoice has moved out of the 'DRAFT' stage, the 'Split' option gets disabled.

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 Xero, click on "Sync to Xero"

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