How to create purchase order from MRP?

Hi,

I have created work order by processing R3482 report, but not able to create purchase order. 

I have gone through following steps, please correct me if I am wrong.

1). Created bill of material item A. components attached B and C.

2). Created routing item A.

3). Created sales order A.

4). Run report R3482.

5). Item A reviewed on P3411.

6). Process item A on P3411, row exit "Process Messages".

7). Systen generated the work order number.

8). But not able to generate purchase order

Please let me know if I have any step.

Thanks and Regards,

Jayanta K.

We are using Material Requirement Planning process.