You can create an invoice in CRM by either cloning an existing invoice or filling in the details in the invoice creation form.
Click the (+) Create Invoice icon in the Invoices module.
Enter the invoice details in the Create Invoice page.
Click Add Line Item in the Product Details section.
Note that you can add up to 200 line items to an invoice.
Once you have added all the product lines, click Add Products to add them to the invoice.
Click Save.
Fields that are needed to be filled before syncing in Xero
Note: make sure you are using the correct tax option (Item base/Total base) you have selected in the Plugin Portal. Click Here for more details
From Zoho, sync can be performed two ways
Manual Sync from Zoho CRM: On your Invoice record simply click the 'Sync to Xero' button in the top right corner to start the syncing process for that particular record. If you cannot see the 'Sync to Xero' option-click the drop-down arrow in the highlighted section for more actions.
Manual Sync from the Plugin Portal: Login to your Plugin Portal. From the right sidebar, go to “Sync > Manual Sync”. Select “Invoices” Module and your desired Sync Mode. From the “Sync Options” section (upper right corner), enable the “After last Synchronization ran” option and click the “Sync Now” button.