How do I sync an existing bill.com account to qbo?

Hi there, @vivian.w.fong.

Let me walk you through the steps on how to sync Bill.com and QuickBooks Online.

1. In Bill.com, hover over the sync menu icon, and then select Setup QuickBooks Online
2. Click Connect to QuickBooks Online.
3. Enter your QuickBooks Online login credential
4. Choose the company in QuickBooks Online to sync to Bill.com.
5. Click Confirm
6. Review that the company selected is correct, then click Authorize. 

For further guidance about the process, click this article:QuickBooks Online Sync - Setup Guide.

Once sync with QuickBooks Online it will only pull in invoices within the last 90 days from the date the sync is set up. If an invoice older than 90 days that need to be synced over to Bill.com, you'll have to click the Edit button and hit Save on the invoice to trigger the update.

Also, I've added this article to know more about limitations when you sync  Bil.com to QuickBooks Online: QuickBooks Online Sync - Notes and Known Limitations.

Tag me below if you have any further questions. I'm here to lend a hand. take good care.