In order to start configuring Magento 2 Alternate Hreflang Extension for CMS Page first thing you have to do is to navigate to Content > Elements > Pages, choose the page you want to add the alternate hreflang to and press the Edit button.

CMS pages edit

Afterward, find the Alternate Store View Localizations section and set the alternative page. In order to do so you have to start entering the page title in an alternative language and choose the corresponding page from the output.

Alternate Tags for Magento 2 CMS Page

Finally, when you've finished don't forget to press the Save button.

Alternate hreflangs configuration on CMS page