If you are only looking for Approve Requisitions and Order from Requisitions initially then please refer the documentation for these apps and perform all the steps mentioned. Approve requisitions requires GBAPP002 600 and Order from Requisitions requires SRA013 600 components in the back end system.
App Implementation: Approve Requisitions - SAP Fiori principal apps 1.0 for SAP ERP - SAP Library
App Implementation: Order from Requisitions - SAP Fiori principal apps 1.0 for SAP ERP - SAP Library
Regards,
Saurabh