Open the customization page and follow the steps below.

Click on Advanced settingsSettings CustomizationsCustomize the system

Select the Entity and click on the form you’d like to add your sub-grid to.

Create the sub-grid to show opportunities.

Give your sub-grid a meaningful name and label, click on ‘Show only related Records’, select ‘Opportunities’ from the list of entities and select a view you’d like to show in your sub-grid.

Add nested sub-grid to show contracts.

Then, proceed to ‘Controls’ section and click on ‘Add Control’.

Choose 'Editable Grid' from the list and click on ‘Add’ button.

Click on these 2 radio buttons:

Then click on this pen button 

In the pop-up window, select ‘Oneflow Contracts’ entity and select ‘Nested Grid View’ from views:

Then click on this pen button: 

Choose ‘of_opportunityid(Lookup.Simple)’ from the list: 

Click on Ok → Ok

That’s it! You created your grid now and should be all set. Position the grid wherever you need to and don’t forget to save and publish your changes. 

View sub-grid on the Account record.

You should be able to see the grid on your form now!   

If you click on the arrow button when viewing your sub-grid you’ll be able to see all contracts that are associated with you opportunity