Titanium backup and TWRP, custom ROM I'm trying to install a new custom ROM on my smartphone and first I need to make a backup. I was wondering, if I use Titanium backup do I need to make TWRP?
I got a bit confused researching online and watching and reading all those tutorials.

I know that to use Titanium Backup I need to root my phone first (I know how to do it), then backup all the data and then do I just install the ROM?
Titanium Backup is a backup utility for Android that backs up your system and user applications along with their data on external storage of your choice. It is a leading Android application that uses root access to pull all your important applications and data from the system partition and stores them for future restoration. Whether your phone is corrupted or flashed with a custom ROM, Titanium Backup can save the hassle to get your phone loaded up just like it was before.

TWRP is an open source, community project. We also have a large support community with many people who are willing to answer questions and help people with their devices either through our IRC channel or on forums like xda-developers.

A custom recovery is used for installing custom software on your device. This custom software can include smaller modifications like rooting your device or even replacing the firmware of the device with a completely custom "ROM" like OmniROM.

A custom Android ROM refers to a phone’s firmware, based on Google’s Android platform. Android is open source and therefore any developer can edit the code, recompile it, and re-release for a wide variety of devices. Users can install ROMs to change a device’s appearance and behavior. ROMs are developed by the Android community, often times by a group of core developers who do this purely out of passion for modding. This means that most are completely free. Custom ROMs are available for phones, tablets, media players, smart watches and almost any type of device running Android.

Notes


You don't need to root for installing twrp and custom rom using twrp, but sometimes required to unlock bootloader to flash twrp. For a complete access and customisation, it requires root access on any ROM.
You can backup all your user app(apps installed from play store) data to any android version, and can restore using titanium backup app, which requires root access. For system apps, both should be in same android version is possible.
many custom roms are available on open source forums like xda. Search the one for your device. Look for some official builds like Lineage os, pixel experience etc. Developer will releases updates as nightlies, which resolves initial bugs, and makes more stable in future.
Gapps for any custom rom can be downloaded from open source platform like opengapps.org.
Rooting may voids your device's warranty. And you must be careful at each step. Because, any mistake can lead to make device brick. Device bricking means, it will continue in repeated on/off state
(Bootloop), won't turn on, boots to recovery only etc, repeated app crash while usage.
You can perform this , 
If you're going to use titanium backup app , then backup all your user app data using titanium backup to SD card or USB stick through OTG. Flash twrp and custom rom. On booting to custom rom, install titanium backup, and restore app data.


If you are using twrp method, you can backup your current rom using pressing backup part in twrp. After flashing custom rom, you can restore data backup which is done by twrp (nandroid) using titanium backup.


A backup of current rom using twrp is recommended, if anything happens in between, you can return to current os without losing any data. Back-up to SD card or USB, don't do it on internal storage, which may get erased.


Source:

Titanium

twrp

Custom ROM