--Create a Login who will own the job 

USE [master]
GO

CREATE LOGIN [ssis_usr] WITH PASSWORD=N'password', DEFAULT_DATABASE=[msdb], CHECK_EXPIRATION=OFF, CHECK_POLICY=OFF;
GO

--Grant the sysadmin role to the previously created login

sp_addsrvrolemember @loginame = N'ssis_usr', @rolename = N'sysadmin'
GO

--Create a user in the msdb system database for the login, creating the mapping

USE [msdb]
GO

CREATE USER [ssis_usr] FOR LOGIN [ssis_usr]
GO

--Grant the following roles to the user

sp_addrolemember N'SQLAgentReaderRole', N'ssis_usr';
GO

sp_addrolemember N'SQLAgentOperatorRole', N'ssis_usr'
GO

sp_addrolemember N'SQLAgentUserRole', N'ssis_usr'
GO

--Create a credential to be used by the proxy - the proxy will execute the SSIS package within the job

USE [master]
GO

CREATE CREDENTIAL [ssis_cred] WITH IDENTITY = N'PRIVATE-E3A52F5\James', SECRET='password';
GO

--Create the proxy, mapping to the credential and provide access to the SSIS subsystem
USE [msdb]
GO

sp_add_proxy @proxy_name=N'ssis_prxy', @credential_name=N'ssis_cred';
GO

sp_grant_proxy_to_subsystem @proxy_name=N'ssis_prxy', @subsystem_name=N'SSIS';
GO