The process is easy. Go to the Google Contacts website: https://contacts.google.com/
Expand “More” and select “Export”.
Export all or some of your contacts, but select vCard format and save to your Desktop.
Go to your Nextcloud website’s Contacts app: https://<your-nextcloud-website>.com/index.php/apps/contacts.
Expand “Settings” and click “Import Into”. In the file dialog, select your downloaded vCard.
Wait for the import to finish.
Now if you are using Google Contacts on your phone, you will want to disable that sync so you do not get duplicated contacts. You can sync your contacts with CalDAV/CardDAV Android app.
You can also delete all of your Google Contacts as well, but I will not include those instructions in case people are just looking at pictures.