How to complete self assessment on this. All expenses, sales are mixed up. We just resgister for VAT last year and needed to include all expenses for the same year.

Hi there, Mindaugas.

I’m here to give information on completing the self-assessment in your QuickBooks Online (QBO) account. 

I suggest manually categorizing your expenses and sales transactions properly. Then reconcile the periods one at a time based on your bank statement then refer to your accountant for further guidance in correcting all the transactions that have been mixed up. 

If you don't have an accountant, I can help you find one. Feel free to visit this page to find an accountant for your business: Find a QuickBooks ProAdvisor.

Furthermore, I am leaving you this article that you can read and refer to on setting up your VAT and how to reconcile and fixed reconciliation in your QBO:

• Set up and edit VAT settings, codes, and rates in QuickBooks Online
• Make VAT adjustments in QuickBooks Online
• Reconcile an account in QuickBooks Online
• Fix the issue the first time you reconcile an account in QuickBooks Online

Feel free to comment below if you have any additional questions about your VAT in your QBO account. I'm always available to assist you.