I am not aware of any step by step guide for the full setup, but I would recommend the following.

1 - Download the Sonos app.

2 - Plug in the Sonos Arc Ultra to mains, and follow the setup steps in the app (it will probably detect the Ultra).  I believe this will walk you through  the Arc Ultra to the TV.  It should also prompt you to add any music or voice services, but if not, you can do that later.

3- Plug in the Sub and follow setup steps in app.

4 - Bond the sub to the Sonos Arc ( I think it will ask if you want to do this.

5 - plug in one of the 300s and follow setup steps

6 - plug in the other 300 and follow setup steps don’t setup as a stereo pair.

7 - Add the 2 300s as surrounds to the Arc Ultra.  You will need to go into settings for the Arc and select the option to add surrounds.

8 - Add music and voice services if not done already.

9 - trueplay tune.

You can setup the surrounds before the sub, it really doesn’t matter.  I would wait to trueplay tune till the end though.  The specific steps for each of these are pretty easy to locate in the app, but if you need help, the support search on the Sonos app is pretty good for finding more details.

If you only have a single Era 300 you can’t create a full home theater system; you need a second Era 300 to create the left and right rear speakers. 

Edited to add: just saw you do have the 2

So as understood, I have to start configuring Arc Ultra first then linking other speakers by adding. Right ? Or other speakers should be configured separately one by one ?

Set up the Arc Ultra first (see below).  If this is a new system, see instructions under “Set up a new Sonos system with Arc Ultra”.  If you already own Sonos products, see instructions under “Add Arc Ultra to your existing system”.

https://support.sonos.com/en-us/article/set-up-your-sonos-arc-ultra

Then set up the Sub. 

https://support.sonos.com/en-us/article/set-up-your-sonos-sub

Then set up your Era 100s (see below under “Add Era 100 to your existing Sonos system”)

https://support.sonos.com/en-us/article/set-up-your-sonos-era-100

After that, go into the setup for the Arc room and choose “Set up surrounds”.  Follow the steps from there. 

https://support.sonos.com/en-us/article/set-up-your-sonos-era-100

So helpful jgatie. Thanks