Yes you fill in every single train you take with Interrail :)
at the top you just mark witch date your Inbound/Outbound journey´s are :)

Yes-ALL trips must be noted/written down and as an extra-to show you use the allotted allowance for going OUT and at the end IN-you fill out those special fields for those days. The extreme theory is even that a 2 min train trip to connect 2 bigger stations should be written down, it varies a bit per country how strict that really is taken.

This forum brims every day with points we know sometimes from 30-40 yrs ago but still keeps those new on the scene puzzled.

Just so that you know. You should never write anything down on your paper pass until you are on the platform ready to board the train. You can never change an entry on a paper pass. If you do that the paper pass will be invalid. 

For the outbound/inbound journey/s you need to enter the date in the appropriate field and then just add the journey/s as any other journey. Each train you take should be a separate journey.  

you can actually cross out (still readable) train details on the traveldiary (If you cant catch a train anymore or deside to take a train longer/shorter as first written down
But you never can cross out a written down travelday at the top section on the ticket Once a date is added there it´s your travelday

Because of that i recommend to fill the traveldiary as late as possible. Sometimes shortly before boarding or directly after i found my seat :)

Thanks everyone for your help it now seems obvious but last night I was getting in a panic about what to write on a bit of paper .