Hi Nischal,

pls check the code that creates a PO from a Purchase Requisition, it will fully guide you, but this needs some work.

You have to install the Purchase requisition module, create a PR and from it create a PO.

The key method for my example is 

make_purchase_order

There are also some other examples, like creating an Invoice from a sales order/purchase order.

For full set of examples that you can wander along them you can search the code for 

.create(

this will guide you more