Is there any link where I could see setting up Authentication by ACS ?

Do you mean cisco ACS, otherwise known as RADIUS server?

I suspect it is Cisco ACS - will below suffice as it may be generally covered and not specific to your use case but I suggest you check the other link on common use case for ACS based on the type of access required e.g. Password-Based Network Access, Certificate-Based Network Access, Agentless Network Access, VPN Remote Network Access, RADIUS Proxy Requests or even building Trustsec
>Step-by-Step Configuration for Cisco Secure ACS
http://www.cisco.com/en/US/products/sw/secursw/ps2086/products_user_guide_chapter09186a008007e6a6.html#wp366022
>Common Scenarios Using ACS
http://www.cisco.com/c/en/us/td/docs/net_mgmt/cisco_secure_access_control_system/5-2/user/guide/acsuserguide/common_scenarios.html#wp1052496

Yes Radius server. Would like to have a simpler documentation other than what Cisco provides. We already have set it up, so want to understand for the purpose of troubleshooting.

There is no simpler documentation than official one.