Tested on:

Xiaomi S2 

EU ported MIUI12.0.8 - android 10

This guide assumes you have a custom recovery & magisk installed. If not then do it first.

We're gonna go in 4 step process:

1)Selinux permissive:

First go to magisk manager & flash Selinux module . Follow the flashing instructions & set Selinux to permissive .

2) Uninstall the original play store:

Go to root >data>app>com.android.vending folder (your device might have some random string name , so find the folder where play store is it might be under the name base.apk/vending.apk/market.apk/ play store.apk) delete the folder using sd maid app.

[Note: I tried bunch of other file managers like fx file manager, xplorer, root browser but for some reason they did not work. ]

3) Delete the basic version of play store from system that automatically reinstalls:

Using sd maid go to system> product > priv-app> Phonesky this is the basic version. Delete it.

[Note: If for some reason you are having trouble with step 2 & 3 then go to your custom recovery mount system partition & follow these steps from custom recovery.]

4) Now if you have a modded apk from Lucky patcher then place it in the Phonesky folder. If not then install the modded play store from Lucky patcher app. Take a backup of the modded play store using any backup app. Then place it in the Phonesky folder. 

Go to app manager if you can Uninstall play store from system app then do it. It will automatically reinstall the modded version . 

5) Set Selinux to Enforcing .

Enjoy.