How do I mark a bank transaction as one instalment of one big bill that is entered in expenses. I can only see reverse, as in one transaction mark off several expenses?

I can show you how to mark those expenses into several categories or accounts, @usersolvig-calderban. 

You can split this bank transaction to mark it or categorize it into several expenses. Let me show you how:

1. Go to the Banking or Transactions tab and go to the Banking menu. 
2. Select the For Review tab.
3. Select the downloaded transaction to expand the view.
4. If the option in the Action column is Match, change the Find match to Categorise.
5. Select Split next to the Add button.
6. On the Split transaction window that appears, provide all the necessary information.
7. Click on Apply and accept once you're done. 

Transactions are now moved to the Categorised tab. 

You can check this article for more information: Categorise and match online bank transactions in QuickBooks Online. 

In case you want to automatically categorise transactions for you, refer to the following article on how to setup bank rules to categorise online banking transactions in QuickBooks Online.

Stay in touch if you have other concerns by leaving a comment below. I'm always glad to help in any way I can.​ Have a great rest of the day.