_{This post assumes you are experienced in installing custom ROM’s and have TWRP recovery already installed on your Phone}

What is a Dirty Install : The difference between a clean install and a dirty install is that here there is no wipe or cleaning up of the existing data.

Why or when would you do a Dirty install: When you do not want to lose existing data on your phone, are too lazy to make a backup of your data 🙂

PreRequisites

• Smartphone with TWRP installed
• A working USB data cable(there are cables for charging only )
• Download the zip file of the custom ROM you plan to install
  
  _{Ensure you have downloaded a ROM which was built specifically for your device.
  Flashing builds incompatible with your device can brick your phone.}

• Take a backup of any important data you have on your phone
• Ensure you have developer options set on your phone

The Steps

• On your phone remove any pattern , fingerprint locks you may have set
  _{This is to avoid any issues during the install process}
• Reboot to Recovery
  
  _{You can also boot into recovery by connecting your phone with a data USB cable to and adb enable computer and typing the below command on a console}
  adb reboot recovery
• Copy the .zip file – the ROM you want to install – from your PC to your phone
  
  You can also do this using the below adb command from the PC console
  
  adb push download.zip /sdcard/
  
• In TWRP main screen tap the Install button
• Browse to the zip file and swipe to install

• Once the install is successful you will see an option to Reboot to System

After this the new OS should boot on your phone. Enjoy !!