Login to your AWS account as an admin.
Click The following link: Setup Costi role which will pre-configure the CloudFormation values for you.
Alternatively use the following link: Setup Costi Read Only which will only give the Costi role read permission.
Click 'Next'. You will be asked for a parameter called ExternalID. You can find that parameter here.
Click 'Next' three times.
Mark the "I acknowledge that AWS CloudFormation might create IAM resources with custom names." checkbox at the bottom of the page.
Click the "Create stack" button.
That's it, you're all done!