Navigate to IAM panel by searching for the IAM service. You'll have a bunch of newly created Cognito roles. Use the instructions on the Amplify docs getting started page and create an Auth and Unauth policy for your new IAM users.
Add each policy you copied with "Attach Policy" to the User Role. Make sure to replace the "example bucket name" with the name of your bucket.
Create the policy, give it a name like "UDTTestBucketRole".
5. Create Example User
In Cognito page click "Create User", enter the username and password. Mark the account as verified. You'll then be prompted with Complete your Sign Up.