In SAP, create or maintaining a Purchasing Organization is quite simple.
Starting with the customization transaction SPRO, go to Enterprise Structure > Definition > Materials Management > Maintain purchasing organization
Here, the list of existing purch. org. will be displayed. Click the New Entries button to create new ones
Enter the organization name and description, and save
A customizing request will be prompted for
A message will confirm the success of the Purchasing Organization creation
In order to maintain an existing organization, in the overview, select the line to update, and click on the update button
You can then change the values as needed