i want to do seamless bookings

The app is there to be used by you, rather than train staff. I’m surprised you found staff willing to help! Well done  

There’s a good guide to using it here:

https://www.seat61.com/how-to-use-an-interrail-pass.htm#how-to-use-a-mobile-pass

You can also buy a paper pass if you prefer. Is not quite seamless. You would still have to write in the train times for every journey, and buy any compulsory reservations. 

honestly i wish it was simple. I all i could figure out was how to make a trip and how to turn the interail on for the day i just could marry the two things
I went to the ticket offices in switzerland and in amsterdam and they researved the train seats for me. on the day i could turn on my passes but could not find the qr code for scanning on the train anywhere. 
The SNCF guy looked at the app for 30 mins and decided it didnt work so didnt bother fining me. In switzerland i couldnt display the qr code so ended up just buying a ticket for that leg of the journey. 

I want to book a train for my son to travel from London to switzerland via lausanne. please tell me all the steps to put it onto his phone for the end of this month. he is 15 so assume he can travel alone but i dont know the answer to that either.

if successful he will do the same trip in december is there a pass i can buy to combine the two? 

Many thanks 

Adam 

How to add journeys and show the barcode is explained here:

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

Especially the section "Getting ready to board the train”. There's also a video.

If that still doesn't help, then please explain at which exact step you are stuck. A screenshot will then also be helpful.

Reservations are entirely separate from the app. The best overview of where and how to book is this page:

https://www.seat61.com/interrail-and-eurail-reservations.htm

If that doesn't work for you, then please mention route, date and departure time of the train(s) you don't manage to book.

I would suggest your son manage his own pass. That way he will become familiar with how it works, and might even be able to help you? The steps are as described in the Seat 61 link above  

As I said, you also have the option of a paper pass.

There are longer passes that might be useful, and a two month pass might be useful to him, but you’ll have to count up the travel days and do the maths yourself as to whether it’s worth it. Note that you can only use the pass in your home country on two days, so if you do a second trip on a pass, you might have to buy tickets to leave the country. This might sway your choice.