how do I create a new email address for my existing Apple ID?

If you want to do this in iCloud see:

Create an email address in iCloud Mail - https://support.apple.com/guide/icloud/create-an-icloudcom-email-address-mmdd8d1c5c/icloud

otherwise create an email account with any provider, then change your Apple ID with Apple:

Please click on the links and read the documents -->

Change your Apple ID - https://support.apple.com/HT202667 - "Follow these steps to change the email address that you use as your Apple ID." Plus special instructions if you want to change your Apple ID to an @icloud.com address. You may also wish to turn off "Find My" before changing information and turn it on again afterward.

"If you try to change your Apple ID to an @icloud.com email address that was created within the last 30 days, you will be asked to try again later.

If you see a message that the email address you want to use isn't available or is already in use, check that you or a family member aren't already using that email address with a different Apple ID. If a family member is using that email address, remove it from their account then try to use it."

I would avoid using an iCloud email address as your Apple ID because should there be problems logging into your account you run the risk of not being able to read the recovery emails from Apple required to fix the problem.

"After you change your Apple ID email address or password, you might need to update your sign-in information with any Apple services you're using" Click on this link to read "What to do after [or before] you change your Apple ID email address or password" --> https://support.apple.com/HT204071

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