i will put the steps i know to create a automatic PO.
1. Maintaining the Material Master & Vendor Master with Automatic PO check box selected. MM01, XK01
2. Material Master should maintain with MRP Views, with reorder point, controller.. etc MM02
3. Maintaining the Source list for a material & for one vendor Check box fix selected. ME01
4. Maintain Info record for the vendor & material ME11
5. Run MRP at plant level for auto PR's MD03
6. Assign & process the PR's at ME57/ME58
7. Creating Automatic PO at ME59N.
Kindly correct me, if i am wrong.