Step 3a: Install a custom rom via recovery
Go to Step 1: Install ADB & Fastboot Driver
Go to Step 2: Unlock the bootloader
If you are not yet in the bootloader, then boot into the bootloader. You can see how to do this in step 2 (Unlock the bootloader).
Download a custom rom of your choice.
You need a so called "vendor_boot.img".
You can get this mostly on the website of the rom but for all roms i have listed here i have added this too.
I use Lineage OS today but this is not important. This works the same with most roms.
Check in advance if your Rom has GApps included or not. I always write this on this page.
Evolution X is a rom with GApps intrigued. So you don't need the next step. Lineage OS does not have any gapps installed so after step 3 you should continue with step 4 (Install Gapps or Magisk).
Install the vendor_boot (includes the recovery).
After that, boot directly into the recovery.
First we reset the device via the "Factory reset" button. Then we go to the "Apply update" menu.
Next, we install the Rom.
This happens with 47%.
And that's when he's done.
If you don't need or don't want to install any GApps or don't want to install Magisk, you are now done and can press the "Reboot system now" button and you are done.
Attention, the first time the boot process can take a little longer.