Thanks for following up with the Community, Paul2410.

When creating a new account in your Chart of accounts, you can make it a subaccount by ticking its Make this a subaccount option. After doing so, you'll be prompted to choose a Parent account for the subaccount to be under.

Here's an image showing this process:
 

Once you've finished entering all of your account's details, select Save to save the account into your books.

I've also included a detailed resource about working with subaccounts which may come in handy moving forward: Create subaccounts

I'll be here to help if there's any additional questions. Have a wonderful Wednesday!