To Create a Parent Account and Sub Account:

1. Go to CRM > Companies.
2. On the company list, click the account you want to make as a sub account.
3. On the Overview > General > General Information tab, Parent Account field, select the account that you want to make as parent account.

After you choose the parent account, you can see in the left side the sub account was under its parent account. There you have the Parent Account with its Sub Account.