An example:
crontab –e
@monthly /your_path/letsencrypt/letsencrypt-auto --config /etc/letsencrypt/cli.ini -d yourdomain.com -d www. yourdomain.com certonly && service apache2 reload
Content of cli.ini:
authenticator = webroot
webroot-path = /var/www/
server = https://acme-v01.api.letsencrypt.org/directory
renew-by-default
agree-dev-preview
agree-tos
email = postmaster@yourdomain.com