Experienced Interrailers/Eurailers always say this: You should NOT activate travel days unless you're very sure you're going to take that 1st train of your travel day. Best wait to activate a travel day until just minutes before your first  journey of that day, when you actually see your train at the platform.

You never know. Delays, cancellations, you can get ill... it's always possible and one doesn't want to squander unused travel days. It's impossible to get them back. 

You can add or remove trains at every moment you want when you use a travel day, but once activated a the travel day you can't cancel them after 0:00 CET of that travel day. 

There's the rule you cited, and I think it's pretty clearly worded. 

If you want to change a travel day, just cancel it or just put some options in "my trip". And wait to add them to the pass.

When you are ready to travel, activate the travel day by tapping "Add to pass" on the trip saved in "My Trip", just before travelling (again, minutes before you board the 1st train of the day) . This can be done in seconds and you minimise the risk of using travel days. 

Best see the tab "My Trip" is your theoretical planning. Adding entries to My Trip doesn't activate travel days et al, it only does it when you tap that grey ledger. Then it generates your ticket for the day. 

As for now, I advise you to keep your planning in "My Trip", but cancel all your activated travel days, if your travel days lie in the future ahead of course. And only tap the ledger "add to pass" minutes/seconds before you board the 1st train of that day. 

Which route you're planning to take btw? Sometimes normal tickets are cheaper than the cost of a pass day. 

I can add to @BrendanDB 's advice that if you have activated your pass, and not yet have started your travel, you should deactivate the pass immediately. You never know what will happen and once the validity of the pass has started, you can't deactivate the pass even if you haven't travelled anything.

There's also a tutorial on how to use a pass here:

https://www.interrail.eu/en/interrail-passes/interrail-mobile-pass/getting-started

Thank you for the guidance. I have not activated the pass. I am waiting until nearer the day of our first journey. Our journeys from UK to Girona and back are planned with reservations made. While we are there we would like to go to Barcelona and back on one day. I have planned another trip for the day out to Barcelona and saved this in My Trip. It says that it is not connected to a mobile Pass. By clicking on the three dots and then on Edit it offers me the option to ‘Connect a Pass to this trip’. When I choose this option it only allows me to choose a Paper Pass and not one of the two mobile passes saved on my phone. I have looked in My Pass where it says ‘Trip connected’ and ‘Next Activate this pass’ which I do not want to do yet. 

You don't need to make a new trip for that, but you can just save some Girona-Barcelona options to the "My Trip" tab of your existing trip, that one with your journeys to/from UK and Girona.

I think only one smecific trip can be connected to specific pass. 

Yes. That is correct. There can only be one Trip connected per pass.

Yes. On the Interrail website it says ‘Each Pass needs its own trip, so if you've got a few Passes on your device, you’ll need a separate trip for each Pass.’ I take this to mean that a Mobile Pass can hold only one trip. If I save more than one Girona-Barcelona date option to my existing trip won’t it take the number of days over the four I am allowed? The Trip already shows three days. Or is it the case that when a trip is added to a pass it appears as separate days and each day has to be activated? 

Your pass days won't disappear unless you effectively activate a travel day with your first journey of that day.

Thank you. So, when I have activated the Pass, will it ask me to activate a travel day?

Can I save journeys on more than 4 days to a 4 days in one month Pass?

Yes, when you press the grey ledger of a journey, saved in "My trip" it will ask your confirmation ("Sure you want to activate a travel day?")

An example of my trip this summer:

Circled in green are non - activated journeys that I've put in the planner. But I actually got a train later, that I activated before boarding. Then it asked if I wanted to activate a travel day. Then it generated a QR code in My Pass, that you'll need to show when inspected. 

The effectively activated and used journeys are circled in red. Note the yellow ledger. 

Excellent. I couldn’t find any explanation as clear as that on the website. I assume I need to be online to activate a travel day and a journey. 

Indeed, you need to be online :)