this is no different from any other sales order for a model item.Only different here is you don't want to create a Workorder in WIP , rather you want to create a Purchase order in PO.
Some Simple setup has required for ATO Model
1.make sure that for ATO Model you set it as Buy Item.
2.Once you set it as buy Item , while creating the configuration in sales Order form (workflow - Line flow generic ATO Model), Model line will process the worklflow activity "create Configuration" and then reach at "Wait for CTO" workflow activity.
3."create Configuration" , will trigger "Line flow - configuration" workflow and in one of it's subprocess "Create Supply" workflow take "Buy" path and process "Buy Ato item Flow" and insert data in PO req table.
3.One PO created/approved/received, it will progress the Configuration line from SHIP_LINE activity and also progress the MODEL line from "Wait for CTO".
You need not to do much setup in the transaction type ,JUST make sure that Basic flows of Line Flow /Model/Configuration sould be assigned to the Transaction type that you are using to create the sales Order.
Just FYI - System will send only Config line to Purchasing nothing else.