How do I assign a customer account as a child/sub account under my main account?

To assign an account as a child account:

Go to CustomersManage.
On the line of the customer that you want to assign as a child account, click Manage.
On this next screen, at the top, in the Account Settings section, you will be able to change the Type to Sub/Child Account.
Do not forget to click Save Changes at the bottom of the page.

Create New Customer Account
If you have not created the customer account yet that will become the child/sub account, and you are uncertain of how to do this, please check this FAQ with steps.