One can download the IDFC FIRST Bank’s mobile App and login using their mobile number and date of birth. To download loan documents, customers can navigate to the FIRSTmoney dashboard by clicking on the FIRSTmoney card in the 'Loans' section. Under Active loans, click on 'View details' and navigate to 'Loan documents' to download loan-related documents. Additionally, customers can access this section to view all loan-related details like due date and payment history.