The main question is why custom roms-:

Custom roms are nowadays widely used because they give us various customisations and increase ram management and speed of the smartphone.

BUT THERE ARE SOME DISADVANTAGES OF CUSTOM ROM-:

-You have to unlock bootloader in order to install them which further voids warranty.

-You dont not obtain further OTA upgrades from the company

-You can also brick your device

AND I AM NOT RESPONSIBLE FOR ANY CASE

as we can see that the advantages of custom rom are more than the disadvantages of installing a custom rom as some smartphones stop obtaining OTA upgrades from the company as the  smartphones become old and company does not bother about them.Moreover the warranty expires over 1 year.

So lets begin the process-

1.The first process is to unlock the bootloader of your smartphones.There are several ways to unlock bootloader in various smartphone. In my case (asus zenfone max pro m2) , the bootloader can be unlocked by the application provided by the asus itself.You can download it here

or just search unlock bootloader tool and download it from official asus website.

2.You have to install the tool in your device and follow the instructions and remember your all data will be erased.

3.You have to install adb and fastboot in your pc. you can download it from here

3.Now you have to flash any recovery in your device(You need a pc here).I prefer TWRP and you can download it here

To flash recovery follow these steps.

------Enable USB Debugging in your device .To enable USB debugging, navigate to Settings >       Developer Options > USB Debugging

------

1. Open the folder where your TWRP Recovery .img file is saved.
2. Then open a CMD window inside that folder. To do that, Shift + Right click on any empty white space inside the folder and then select Open command window here.
3. Connect your Android device to the PC. Type the following into the command window to boot your device into bootloader/fastboot mode:

   adb reboot bootloader

   └ If your asks for permission to “Allow USB debugging”, tap OK.

4. Once your device boots into bootloader mode, type this into the command line.

   fastboot flash recovery twrp-2.8.x.x-xxx.img

   └ Here modify twrp.img with the name of your TWRP recovery .img file.

5. Once TWRP is successfully flashed on your device, type this final command to reboot your device.

   fastboot reboot

4.Your device now have a recovery. now you only have to download your favourite custom rom and flash it in twrp.

-----reboot your device in recovery mode

       you have to paste the rom in external storage for eg. sd card or an otg usb.

       select the file and flash it.

       wipe cache/dalvik and reboot.

NOW YOU HAVE SUCCESSFULLY INSTALLED A CUSTOM ROM.