Once you have created a Custom Field in QuickBooks Online and linked it to a field in LeanLaw, you will need to setup a Custom Report in QuickBooks Online.
Here is a QuickBooks Online walkthrough of how to create a Custom Report by adding your Custom Field:
The Tutorial also has links at the bottom that include the QuickBooks Online Reports that may be customized.
How to attach a Custom Field to a Report in QuickBooks Online:
Here is an example of a Custom Report showing a Custom Field: