S3 Cross Account Role Assumption

Cross account roles with AWS

I recently spent far more time than I would care to admit trying to work out how to use cross account roles to access an S3 bucket that had many files written by different AWS accounts resulting in a *soup* of ACL’s. A large part of this time burnt was because I couldn’t find an idiots guide to cross-account roles, so here is what I hope to be a thorough explanation of how to use cross-account roles to access an S3 bucket in a different account.