The most important one is already mentioned:
- Add the mentioned lines to e.g.
/etc/apache2/sites-available/default-ssl.confor the config you used to set up the ssl certificates etc. following your guide.
Btw:
- The guide gives steps to enable self-signed certificates. I strongly recommend to switch to letsencryt/certbot. The benefit is, that it automatically enables a stronger ssl configuration and users won’t face the “untrusted” warnings within their browsers when visiting your cloud.
- Just
apt install python-certbot-apacheandcertbot --apache --rsa-key-size 4096and type in your domain info… done. It asks also if it should enable HSTS for you. - The guide offers to move data directory post-installation. Note that this can cause problems that way and it is strongly recommended to do it already inside the web installation wizard. So the database is not set up wrong first. I mention all about that here: Howto: Change / Move data directory after installation
- Also the way the guides does, Nextcloud is installed with SQLite database. It is strongly recommended to use MySQL or better MariaDB for it on production systems. You need to set up the database first or use an automated occ installation command. But more on this if you need.
- To be true, there are better guides out there and here in the forum or docs.nextcloud.com
. If you have login there for comments, you could mention the hints there 
.