HAPROXY integration with Varnish and Apache

I have use HA Proxy for Application Load Balancing but I have not used with Varnish and Apache server. We are using CLIENT - HAPROXY - VARNISH - 2 WEB SERVER set up and I need help to configure HAPROXY and VARNISH. Please suggest what needs to be configured in Apache.

It is for hosting an E-Commerce web site, please suggest if I have to make any specific settings for optimal usage.