If you’re unaware, the bottom tray can actually have a maximum of 5 apps. To add one more, you just need to drag an app from the home screen to that area.
If 5 apps aren’t enough, then you can create a folder to group your most-used apps and put it to the bottom tray. It’s that easy !
Note: The steps here were written for MIUI V5, based on Android 4.2.2 on a Redmi (HM1W). If you’re having a different model of Redmi or other version of MIUI/Android, there’s a possibility that the steps and screenshots will be different.