1. Install mod_ssl and python

yum -y install mod_ssl python26-virtualenv

2. Change into /opt and clone letsencypt

cd /opt

git clone https://github.com/certbot/certbot letsencrypt

3. Run letsencrypt

/opt/letsencrypt/letsencrypt-auto --debug

Select relevant options when prompted.

4. Setup auto-renew

Auto-renew certificates every Sunday at 6pm:

crontab -e
# Renew Let's Encrypt certificates at 6pm every Sunday
0 18 * * 0 root (/opt/letsencrypt/certbot-auto renew && service httpd restart)