Before installation, make sure the device is rooted and download adb/fastboot, the correct twrp image and the correct viperone zipfile for the phone.
It's easiest to just put it all in one folder so no entering long foldernames!
Extract adb/fastboot in a folder and note down the foldername.
Next, open a commandprompt in administrator mode.
Last, attach the phone to the USB port on the pc.
Once that is done, you can verify that the phone is actually responding to ADB:

cd (folder where all the files are.. i.e. c:/phone)
adb devices

You should see something like:

List of devices attached
HT.... device

If not, it is likely that usb debugging is not activated.
You activate this by first activating the developer options in the settings menu by going to settings -> about -> software info and tap the build number option 7 times.
Then go back to settings top menu, find developer options and activate usb debugging.

Once adb seems to work, run the following commands:

cd (folder where all the files are.. i.e. c:/phone)
adb reboot bootloader
(wait until the phone reboots and windows again detects a usb device)
fastboot flash recovery (twrp filename)

If this is succesfull, it will tell you. If not, check the command you typed.
Now disconnect the phone, use the volume up/down to select hboot and press power.
Now, select recovery and klick power again.
TWRP should now boot and show up.

The navigation bar can be modified in 2 ways:

Using system ui tuner. For this, pull down the top app drawer, and long-press the setting icon. It should show that the system ui tuner is enabled.
- Go to settings, then find the system ui tuner item.
- Select Navigation bar
- Edit the navigation bar at will.
- To make a custom item on it:
  - Download an icon at https://www.iconsdb.com/icon-sets/snow-icons/
  - Store it on your phone in a folder Navicon (or whatever you prefer). Make sure the name has no - or other non-character sings in it!
    Select the PNG format at either 48x48 or 64x64 size.
  - Select add option. Then from the menu, select 'key'
  - Look up the key you want to be activated if you click this icon.
  - Select the image you just uploaded.
  - Drag it to the spot you want it.
Using ADB shell.
- Open your command prompt and go to the folder where ADB is in.
- Attach the phone and ensure that ADB can find it (adb devices)
- Enter "adb shell"
- The phone should now be on your homepage, then enter "su" and grant it supersu rights.
- First enter: "settings list secure"
- Look if there is already a key called: sysui_nav_bar
- If there is, take a note of the setting
- To set the navigation bar, use: settings put secure sysui_nav_bar "... setting string ... "

Setting string for the navigation bar
This string defines 3 'zones' on the navigation bar: left, center and right.
These are set in the string using ; as seperators. (i.e.: left;center;right).
Each zone can have multiple buttons defined, each seperated with a , (comma).
Each button has a define name to use for easy access.

An example is: spaces,back;home,spacer;recent,spacer

A customized button can be defined also by using the format: key(<keycode>:file:///sdcard/Navicons/(filename png))
Again, download an icon from https://www.iconsdb.com/icon-sets/snow-icons/ and save it on the sdcard.

As an example, to simulate a powerbutton press from the navigation bar: key(26:file:///sdcard/Navicons/power.png)

a complete setting command could be for example:

settings put secure sysui_nav_bar "back;recent;key(26:file:///sdcard/Navicons/power.png),home"

(yes, that triple slash is mandatory!!)

If for some reason you messed things up, the UI will crash on you...!!!
The thing to do to restore the UI to a working state:

adb shell
settings delete secure sysui_nav_bar

with that, the navigation bar is reset to it's default setting (and for some reason.. your background is cleared also..).

Finding the correct keycode
This is easiest to find using method one, using the system ui tuner and defining a key. By using the adb list command you can see the setting for sysui_nav_bar in key(..)
If you can't find it, there is a list on elementalx.org, keycodes