git clone https://github.com/letsencrypt/letsencrypt
cd letsencrypt
./letsencrypt-auto

If this command doesn’t work, you can try the following manual way

./letsencrypt-auto certonly --email [email protected] --webroot -w /var/www/vhosts/yourdomain.com -d yourdomain.com -d www.yourdomain.com

You can add multiple domains under the same certificate

./letsencrypt-auto certonly --email [email protected] --webroot -w /var/www/vhosts/yourdomain.com -d yourdomain.com -d www.yourdomain.com -w /var/www/vhosts/yourdomain2.com -d yourdomain2.com -d www.yourdomain2.com

Update your Apache configuration to use the new certificate:

SSLEngine on
SSLCertificateKeyFile /etc/letsencrypt/live/yourdomain.com/privkey.pem
SSLCertificateFile /etc/letsencrypt/live/yourdomain.com/cert.pem
SSLCertificateChainFile /etc/letsencrypt/live/yourdomain.com/chain.pem

Check domain:
https://crt.sh/?q=%25yourdomain.com

Renew the certificate with a script set in a cron (set every 2 months – certificate expires every 3)

# run on the 1st of Odd Months at midnight | Renew certificates
0 0 1 1-11/2 * root /root/letsencrypt/renew.sh >/dev/null 2>&1

renew.sh script:

#!/bin/bash

[email protected]

LOG=/var/log/letsencrypt/renew.log
SENDMAIL=/usr/sbin/sendmail
CMD="renew"

/root/letsencrypt/letsencrypt-auto $CMD > $LOG 2>&1

if [ $? -eq 0 ] ; then
    service httpd graceful
else
    echo -e "Subject: [letsencrypt] SSL automatic renewal FAILED\n$(cat $LOG)" | $SENDMAIL $MAIL
    exit 1
fi

Sources:
https://letsencrypt.org/getting-started/
https://www.a2hosting.com/kb/security/ssl/securing-your-site-with-a-lets-encrypt-ssl-certificate