Before doing the following you should be sure that your bootloader is unlocked.
Download twrp-TheStrechh-RX.img from HERE, and platform tools from HERE
Extract platform tools in C:\ and copy boot.img in folder.
Open terminal (CMD) in local folder and paste this command fastboot boot twrp-TheStrechh-R2.img , then click enter.
Device boot in TWRP interface go to Advance and select Flash current TWRP and confirm.
Done, you have TWRP installed fine, now can reboot system.
First option
Download ROM, then go to Install -> select your ROM.zip ->Confirm.
Back to TWRP Home, Go to wipe -> format data -> wipe cache from advanced wipe (For clean flashes only. If you are just updating, ignore this step).
Back to TWRP Home, Go to reboot -> and change active slot. If active slot was A, select B. Or if active slot was B, select A. There is an indicator there shows active slot.
Back to TWRP Home, go to Advanced -> flash current TWRP . If you forget this step, you'll lose TWRP after reboot.
Now reboot system and enjoy.
Second option
Boot in TWRP -> format data -> yes
Install the ROM with adb sideload -> adb sideload rom.zip in adb termianl.
Back to TWRP Home, go to Advanced -> flash current TWRP . If you forget this step, you'll lose TWRP after reboot.
Reboot recovery and flash the gapps of your choice.