The first step related to the backend setting extension is:-

Adding Warehouse
On Magento admin Panel, Manage Warehouse > Add Warehouse

Adding a new warehouse and editing details of the existing warehouse can be done here, once all the changes are done click on save warehouse.

Assigning Warehouse
Once the warehouse is added, the products have to be assigned to the related warehouse. In order to assign products to the warehouse on Magento Admin Panel > Catalog > Manage Products.