Where should I start in order to install a custom rom?

[4ur0r]

I think the title is pretty much self explainatory.
I'm about to buy this phone coming from an old Nexus 4 which is stupidly easy to mod but I heard that Xiaomi phones aren't that easily moddable.
What I would like to do is install twrp and Lineageos rom, on my Nexus 4 I would just plug the phone on my pc, start fastboot and flash the recovery, but on the Xiaomi phones I don't really know where to start.

 

[NexusPenguin]

Hi,

Before you even buy the phone, please create an account on the Xiaomi forum and ask for an unlock code here :
http://en.miui.com/unlock/

It may take up to 10 days to get it !
But I think you can ask for that code even before you buy your device

Once you get it and you have unlocked your phone, installing TWRP and a custom ROM is as easy as with any other Nexus Device

Regards.