How do I associate a check I wrote with a contractor I've created? I wrote the check before I created the contractor's personal details in QBO.

Welcome to the Community, @nergin1.

To associate a previously created check to a specific contractor/vendor, all you need to do is to open that transaction and assign a name. Here's how:

1. In QuickBooks Online (QBO), go to the Expenses menu at the left pane to get to the Expenses page.
2. Locate and select the check to open it. 
3. In the Payee drop-down, choose the name of the contractor/vendor.
4. Hit Save and close to complete the process. 

Just in case, I'll add these articles for future reference:

• How to write checks.
• How to print checks.

Please let me know if you have any other concerns or issues, and I'll get back to you right away. I'm always here to answer them. Enjoy the rest of the day!