You can use DSCC to perform this task. For information, see Directory Service Control Center Interface and the DSCC online help.
Create your own schema definition file, such as 98mySchema.ldif.
The syntax of definitions in the schema files is described in RFC 4517.
Read When Creating Custom Schema Files before creating custom schema files.
(Optional) If this server is a master replica that sends updates to other servers, copy your schema definition file to each server instance in the replication topology.
The replication mechanism cannot detect any changes that you make directly to the LDIF files that contain the schema. Therefore, your changes are not replicated to consumers even after restarting the master.
Restart each Directory Server instance to which you copied your schema definition file.
Your changes take effect when the servers restart and thus reload schema definitions.