The first walkthrough is that an AWS account owns a bucket and has an IAM user in the AWS account. The scenario is as follows:
The IAM user has no permissions and needs to be granted permissions to operate any tasks on the bucket
The bucket owner and the parent account are the same
The AWS account can use a bucket policy, a user policy or both to grant its user permissions on the bucket
Tip
AWS IAM (Identification and Access Management) is to control access to AWS resources. For more information about IAM, see http://docs.aws.amazon.com/IAM/latest/UserGuide/introduction.html.
The following diagram shows the relation between an AWS account, an S3 bucket, an IAM user, a bucket policy, and a user policy for this walkthrough:
To summarize, what you need to do is: