Dear,
1. Create PO in ME21N ( on which date Customer has given PO to Company ) .
2. Create SO in VA01 and Delivery in VL01N and Billing in VF01 .
3. Create RE in VA01 ,Returns Delivery LR in VL01N and RE ( credit memo for returns) in VF01 .
Note : Steps 1 and 2 create in past date as when Customer has given and taken Goods and Step 3 create present dates .You need to create one dummy material and run the process with stock .
If you have any doubts please let me know .
Thanks,
NK