Switching on the Serial and lot tracking in BaaN is very easy part. However building the system to track the lot and trace them back from sales order to purchase order is bit difficult or rather tricky part as it would need information like:
1. how many 'stages' of production are there from RM to FG,
2. how many RM need to be tracked / traced
3. whether you have bar codes / similar technology implemented or the data entry is manual?
4. Volume of production and many more.
Considering the complexity, I would suggest that you first get all the information about the points, study the material available in BaaN help or other websites and implement it. It's very difficult for somebody to tell you complete process on this forum.