Here is the link to the latest version of the Splunk App for AWS (https://splunkbase.splunk.com/app/1274/#/overview) that has made many improvements in ease of use regarding setting up various AWS data inputs ex. CloudTrail, Config, VPC Flow Logs, CloudWatch, S3, etc.
The latest installation guide for the Splunk App for AWS not only provides installation steps for the app, but also guidance on setting up the AWS services and IAM permissions for the Splunk App for AWS.
Splunk App for AWS Installation Guide
http://docs.splunk.com/Documentation/AWS/latest/Installation/Abouttheapp
Configuration guidance on AWS Services for the Splunk App for AWS
http://docs.splunk.com/Documentation/AWS/latest/Installation/ConfigureyourAWSservices
Configuration guidance on AWS IAM permissions for the Splunk App for AWS
http://docs.splunk.com/Documentation/AWS/latest/Installation/ConfigureyourAWSpermissions
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