For configuration on ASA ,

http://www.cisco.com/en/US/docs/security/asa/asa72/configuration/guide/site2sit.html

For configuration on the Linksys router

http://www.homenethelp.com/vpn/router-linksys.asp

The options on the VPN page may be a bit different depending on the firmware.

Make sure you have the same pre-shared key , Encryption, Authentication settings on both sides.

Also, on the Linksys box,

Under Remote Secure Group > select Subnet and then provide the LAN IP subnet that is behind the ASA.

Under Remote Security Gateway > select IP address and provide Public IP of the ASA box.

PFS > keep it disabled.

Hope this helps.