From Portal:
1. Sign in to the AWS Management Console and open the Amazon S3 console at https://console.aws.amazon.com/s3/.
2. In the Buckets list, choose the name of the bucket that you want to create a bucket policy for or whose bucket policy you want to edit.
3. Choose Permissions.
4. Under Bucket policy, choose Edit. This opens the Edit bucket policy page.
5. On the Edit bucket policy page, explore Policy examples in the Amazon S3 User Guide, choose Policy generator to generate a policy automatically, or edit the JSON in the Policy section.
Here add a policy statement with required principals and appropriate condition.
6. In the Policy box, edit the existing policy or paste the bucket policy from the Policy generator. Make sure to resolve security warnings, errors, general warnings, and suggestions before you save your policy.
7. Choose Save changes, which returns you to the Bucket Permissions page.

From Command Line:
To add a policy with required permissions and appropriate condition as needed, run:

aws s3api put-bucket-policy –bucket BUCKET-NAME –policy file://policy.json

References:
1. https://docs.aws.amazon.com/cli/latest/reference/s3api/put-bucket-policy.html
2. https://docs.aws.amazon.com/AmazonS3/latest/userguide/example-bucket-policies.html