First of all, read this section for use of mobile passes.

Booking reference number for what? If it is seat reservations for a train it should have details of train time, train number, carriage and seat numbers. If this is what you have then it, along with an activated mobile pass is what you need to travel. Passes and reservations are not linked, they are separate but for reserved trains both are necessary.

When you activate your mobile pass it asks for a start day which is automatically made your first travel day and reduces the remainder by one. You can add as many trains as you want for that day.

There is advice later on when to do this. 

Unless you are travelling today I advise you to disable your pass until your first travel day. It will restore your 4 days.

I assume you are now aware of the difference between trip, journeys and travel days, but just in case here is a short summary:

Each pass is loaded into the rail planner app. You only need to go to step 2 at this time.

In the app you will see buttons on the bottom - the important ones being Trip and Pass.

You create one trip per pass and this becomes your diary for the whole period of your pass. You add journeys into here, but like any diary you can add, change or remove at will at  any time before travelling.

Your journeys may be a single train or several trains incorporating changes of train. The Trip will show all the individual trains with a tab to load them to your pass (Do not switch this until ready to board each train.

Then you can go to the pass tab and press the see ticket button. This creates a QR code that is your ticket that you show when requested, along with any reservation details you may have (separate process).

Every time you transfer the first train of a day from Trip to Pass it creates a new travel day.

Do not do this until you are sure you will be travelling on that train, ideally when it is in front of you.

There are plenty of guides on reservations which complement your pass but are never linked in the pass.