Hi Vanessa,
Fiori app works fine if the ERP backend is configured.
I guess your question is how to implement the standard leave request approval in ERP.
You can work with your colleague who knows HR.
You can find many documents in SCN. Search result by "HR leave request workflow".
Also take a look at Fiori HCM Apps Pre-requisites by Tejas Chouhan.
Regards,
Masa / SAP Technology RIG