I am looking for the steps to configure sso between third party and SharePoint 2010 application. My implementation constraints are:-
1. SSO has to be configured between SharePoint 2010 and third party application(We don't have control on this application).
2. Implementation would be IdP initiated SSO. IdP will send assertion tokens to sharepoint application.
If I understand correctly you intend to setup Federation between SharePoint 2010 (with Siteminder) and the third party application, having the third-party application as the IdP.
CA SiteMinder® Agent for SharePoint lets you protect your SharePoint resources using CA SiteMinder® web access management capabilities. The federation capabilities allow partnering organizations to trust and share digital identities and attributes of employees, customers, and suppliers across trust domains. These trust domains can exist within one organization or between different organizations.
These federation capabilities also provide single sign-on across partner sites. The CA SiteMinder® Agent for SharePoint provides a custom CA SiteMinder® solution which issues claims and packages claims into security tokens, used to validate and access SharePoint resources.
For details, please refer to the "SharePoint Claims-based Authentication" chapter in the SharePoint Agent documentation:
Agent for SharePoint Integrated Documents