1] Remove the Sim Card(s) from the Phone and Power it OFF.
2] Boot your Phone into Bootloader Mode by long pressing Volume Down + Power Button.
3] Once you're in the Bootloader Mode, locate the twrp.img on your PC , open a command prompt there , type in fastboot boot twrp.img , for example : fastboot boot twrp-3.4.0.1.img
4] Once you're booted into the TWRP , Click on WIPE > FORMAT DATA > TYPE YES , If it's done , Return to the Home Screen of TWRP , Click on REBOOT > BOOTLOADER.
5]Boot the twrp.img again like you did earlier , Once you're booted , Click on WIPE > ADVANCED WIPE > tick "INTERNAL STORAGE , SYSTEM , DATA , CACHE" , basically everything except "MICRO-SD and USB OTG" , After wiping , return back to the Home screen of TWRP , REBOOT > BOOTLOADER.
6] Boot twrp.img once again , like you did earlier , Now locate copypartitions.zip , rom.zip , twrp.zip and gapps.zip on your PC and move it to your river , Click on INSTALL > locate the files which you moved to your river ; possibly in /sdcard/ <here>.
7] [FLASHING]
i] First, flash copypartitions.zip , After flashing > Home Screen of TWRP > REBOOT > BOOTLOADER.
ii] Boot twrp.img once again , INSTALL > Now flash rom.zip along with twrp.zip , you can select multiple zips. > Home Screen of TWRP > REBOOT > RECOVERY.
iii] [SKIP THIS STEP IF YOU DONT WANT TO INSTALL GAPPS] ; Once you're into TWRP , INSTALL > gapps.zip , if it's done > Home screen of TWRP > REBOOT > BOOTLOADER.
8] We're Almost done! [IF YOU'RE A GAPPLESS GUY , I SUPPOSE YOU ARE IN TWRP RECOVERY , BOOT INTO BOOTLOADER!] Open Command Prompt on your PC and type fastboot -w , then just power your device OFF , disconnect the cable , manually power it ON , you've successfully installed a custom rom :))
9] Once you're booted into the rom , insert your Sim Card(s) , and the phone will prompt which sim to use for data.