Howdy!
Simplified answer. just
1. Create/configure the new branch/plants + business units + accounts (if you have accounts by BU) etc, just like you have done for the existing branch
2. Create item/branch records in the new branches
3. Transfer the inventory from the old branch to new branch(es) using IT document type
depending on your setup, there may be some AAIs and flexible accounting rules that need to be revised + advanced pricing and preferences setup. Your issue description is not really adequate to give a meaningful answer except steps 1-3
Do you also have different companies involved? Or need to migrate historical transactions to the new branches? (in which case things get more complicated)
Also, if the physical warehouse is still the same and you use actual physical locations as your branch/plant locations, things can get messy because the same physical location should not be defined in more than one branch if you want to do any kind of location control or manage the physical locations/ownership. I this case you would need to reserve location ranges for each of the branches which will make your warehouse less effective from the location utilization perspective
Hope this helps
Aarto