Step 1 : Copy the Marshmallow 6.0.1 Custom Rom and Gapps in your SD card

Step 2 : Boot into Recovery

• Switch off your device
• Now press Volume Down + Power Button together for 2-5 seconds until you see the bootloader
• Connect the USB to your PC/Laptop and then flash TWRP recovery (read the guide here)

Step 3 :  Make full wipe including “system” (data, dalvik, system, cache)

Step 4 : Now go back and click install and then click one level up

Step 5 : Flash the ROM and then the Gapps

Step 6 : Reboot (the first boot can take up to 2-5 minutes)