Self-tuning memory is activated on database creation. Unless you are using a partitioned database, we recommend you leave the settings to automatic. You can also run the configuration advisor and do additional tweaking on parameters.
Capacity planning is crucial for performance. If you need to deactivate self-tuning, you will need some planning and preparation in order to split your server's memory into chunks for each instance. 4-8 GB per processor should be enough for most applications.
Each instance will have to go through a design process, so for each instance, we suggest you proceed from the top, at the partition level, the instance (database manager) level, and at the database level. We can consider, next, the agent's private memory: