1 - Go in the DNS settings for your domain
2 - Create a new A entry with hostname = [subdomain] and IP = [server_ip]
3 - On your server create a new Apache's virtualhost site on etc/apache2/site-availables/[subdomain].[domain].conf
4 - Paste in the file the following code
<VirtualHost *:80>
ServerAdmin admin@example.com
ServerName [subdomain].[domain]
ServerAlias www.[subdomain].[domain]
DocumentRoot /var/www/html/[folder]
ErrorLog ${APACHE_LOG_DIR}/error.log
CustomLog ${APACHE_LOG_DIR}/access.log combined
</VirtualHost>
5 - In terminal, enable the site with: sudo apache2 a2ensite [subdomain].[domain].conf
6 - Reload apache with: sudo apache2 reload