To create a DB Subnet Group on AWS, follow these steps:
Access the AWS Management Console.
Locate and select the Amazon RDS service.
In the navigation menu, choose Subnet groups.
Select Create DB Subnet Group.
In the Create DB Subnet Group interface:
Set a name for your subnet group in the Name field.
Enter a description for your subnet group in the Description field.
Choose the default Virtual Private Cloud (VPC) or the VPC you have created.
In the Add subnets section, select the Availability Zones (AZ) containing the subnets from the Availability Zones section, and then choose the subnets from the Subnets section.
Press the Create button to complete the DB Subnet Group creation process.
Note: If you have enabled Local Zone, you can select an Availability Zone group on the Create DB Subnet Group page. In this case, select the Availability Zone group, the corresponding Availability Zones, and Subnets.
Once completed, your new DB Subnet Group will appear in the list of DB Subnet Groups on the RDS console interface. You can select the DB Subnet Group to view details, including the list of subnets associated with this group, in the details section at the bottom of the window.