Disclaimer:
Some steps or configurations described in this post may incur charges when using AWS services. It is your responsibility to review AWS pricing and monitor your usage. After testing, ensure that all resources are properly deleted to avoid unexpected charges. Always use the AWS Free Tier where applicable and test responsibly.

1. Download Oracle installation media (eDelivery, Oracle Support)
V982063-01(V982063-01.zip) : Oracle Database 19.3.0.0.0 for Linux x86-64, 2.8 GB
Patch 36912597(p36912597_190000_Linux-x86-64.zip): DATABASE RELEASE UPDATE 19.25.0.0.0
Patch 6880880(p6880880_190000_Linux-x86-64.zip) : OPatch 12.2.0.1.44 for DB 19.0.0.0.0

Additional patches could be included if needed.

2. Create S3 bucket to store the installation media. I used a bucket without public access, in the same region where DB will be. My bucket’s name is ora2aws.

3. Upload files to the S3 bucket. It should looks like

4. KMS Key
A customer managed symmetric encryption KMS key is required for RDS Custom. Either create a new customer managed symmetric encryption KMS key or use existing one.
Creating key is simple and can be done from KMS service from AWS Console. 
!!! Do not forget to add the following actions to the key policy – kms:Decrypt*, kms:GenerateDataKey*
!!! They are not there and without them creating RDS Custom DB will fail

5. Go to RDS in AWS Console and from the right pane select Custom engine versions -> Create custom engine versions.
I will use the following data –
Engine type – Oracle
Edition – Oracle Enterprise Edition
Architecture settings – Oracle multitenant architecture
Create new CEV is preselect
Engine version – Oracle 19
Custom engine version name – ora19cev
Description – provide some meaningful description
AMI ID will be left empty
S3 location of manifest files – browse S3 and select your bucket with the installation files. In this case s3://ora2aws



CEV manifest – you can either upload your manifest(JSON file) to some S3 bucket or paste it directly here. I will paste it here –
{
    “mediaImportTemplateVersion”: “2020-08-14”,
    “databaseInstallationFileNames”: [
        “V982063-01.zip”
    ],
    “opatchFileNames”: [
        “p6880880_190000_Linux-x86-64.zip”
    ],
    “psuRuPatchFileNames”: [
        “p36912597_190000_Linux-x86-64.zip”
    ]
}

KMS Key – provide customer managed symmetric encryption KMS key

Some Tags could be added – it is up to you. Finally, click on ‘Create custom engine version’ button.

If you get ‘The service-linked role is in the process of being created. Try again later.’ – just wait few minutes and try again.

CEV creation took ~2 hours and 20 min.
It creates 2 EC2 snapshots (Seed Database Snapshot and Binvol Snapshot), each 25G, they can be seen under EC2->Elastic Block Store->Snapshots

Reference – Setting up your environment for Amazon RDS Custom for Oracle

Happy clouding!