Purchase order approval Workflow

Hi all, I'm trying to create a workflow to manage my PO. I have several users who has a title. Ex: Plant managers A, Plant managers B, ..., store KeeperA,etc..

I want to be able to create a simple workflow where I can define different approval levels .

Ex: If The PO approval amount is > 5000$, then one person who is a plant manager  should approve. However i need in my case, a user A should only be approved by Plant manager A, etc...

If someone can give me some hints to have a create a simple workflow, it will be appreciated.

Thanks