Installation

IF INSTALLING FROM STOCK
-Download any custom recovery you wish And Vbmeta
-Install Odin And Samsung USB Drivers
-In Your A21s: Tap The Build Number A Few Times(dev options should be unlocked)
-In Developer Options,enable OEM Unlocking
-Now Power Off Your Device
-Hold Volume Up And Down, Then Plug Your phone in your computer
-in download mode,hold volume up until a popup appears saying "Unlock Bootloader?" Hold The Volume Up,now your data is gone
-even if you now try to flash custom recoveries and other stuff, they will FAIL,
This happens because KG State Is Prenormal, How To Turn It To Checking?
-In Developer Options: Disable Auto Updates
-Change Date And Time To 2 Weeks In The Future
-Now Connect In Internet And Check For Updates
CONGRATS! KG State Is Now Checking
This Means That You Can Flash Anything You Wish
After Turning Kg State To Checking:
-Power Off Device
-Enter Download Mode
-Press Volume Up
-Flash Custom Recovery(Preferably TWRPV2.tar Found In Gdrive Folder) In AP Box And Vbmeta(use the vbmeta from the Gdrive Folder) In Last Box
-Turn Off Auto Reboot And Press Start
-After Its Done Hold Volume Down And Power Button Untill Screen Turns Black And Then Hold Volume Up And Power Button
IF INSTALLING FROM ANOTHER ONEUI ROM OR ALREADY HAVING CUSTOM RECOVERY
-Download zip
-Download Repartitioner
-In TWRP, Flash repartitioner.zip(!THIS WILL WIPE YOUR DATA,MAKE SURE YOU HAVE A BACKUP!)
-Reboot Recovery
-Format Data and wipe Dalvik\ART,Cache
-Reboot To Recovery Again
-Flash The Rom.zip Now
-Flash Atlas Kernel
-Wipe Data
-Reboot System
-DONE!