How to Lock Bootloader for K20Pro?

[DrXeR]

I recently bought a used k20pro premium edition, it is running MIUI 12 Gobal stable ROM, it receives regular updates , does not have custom ROM and doesn't seem to be rooted. Fully functional Playstore (though it says play protect not certified)
It didn't support Google Pay, so I checked and found that Bootloader is unlocked.

I am a rookie, and would appreciate if someone can give a stepwise guide to help me lock the bootloader again so I can use Google pay WITHOUT bricking the phone, or losing my Stable ROM?

Thanks and regards.

 

[95Viper]

Hope this thread at XDA helps: How do I relock the bootloader and go stock?