These steps are valid for Ubuntu 18.04.
Your webservice in sitedirectory has to be configured already.
add-apt-repository ppa:certbot/certbot
apt install certbot python3-certbot-apache
cd /var/www
cd sitedirectory
mkdir -p .well-known/acme-challenge
cd .well-known/acme-challenge
certbot –apache -d webservercommonname