After payment completion, return to Elite Game Portal and the download will begins automatically. If you encounter the warning message like this, please press OK to proceed. Then, click on the APK file to start the install on your device.
Open SETTINGS and navigate to UNKNOWN SOURCES option (under Applications or Security depending on your device). If Unchecked, tap the checkbox and then tap OK on the confirmation popup. (If UNKNOWN SOURCES is already checked, you can skip this step)