At this point letsencrypt has created your certificate in /etc/letsencrypt/live/example.com/cert.pem, as well as your private key (privkey.pem) and a file with both your certificate and any intermediaries (fullchain.pem).
The remaining steps are independent from Let’s Encrypt, so basically any guide for installing SSL certificates on apache would work. The simplest version - and I don’t really recommend this for production - would probably be something like this:
<VirtualHost *:443>
# ...
SSLEngine on
SSLCertificateFile /etc/letsencrypt/live/example.com/cert.pem
SSLCertificateKeyFile /etc/letsencrypt/live/example.com/privkey.pem
SSLCertificateChainFile /etc/letsencrypt/live/example.com/fullchain.pem
</VirtualHost>
Mozilla provides a more complete sample configuration here:
https://wiki.mozilla.org/Security/Server_Side_TLS#Apache