Attention: departure and/or destination station has to be located in Germany.
Visit this website of the Deutsch Bahn (see link bellow)
Fill in the journey you want to travel, the date and time.
You will see a list of trains - select the one you want to buy a reservation for.
The night trains are usually called CNL trains.
Sometimes the system asks then for your age - it is not important for the reservation, but fill it in - there is also an extra field. Keep it with "no reduction". Press refresh and do it again like at step (4).
Now you see the list of the different types of seats, couchettes and beds you can book. But still at the normal high fares. Select the button at the bottom beside the red one called: Book only extra charge
On the next page select Pass offer (InterRail, Eurail, other pass offer) and then: Continue
Select the type of seat/bed you want and press: Book selected offer
Reserve here online!
YES, I want to support Raildude and all the information with my purchase via this link.
Buy the reservation at a railway station. Note: in some countries you have to pay a service fee at the ticket counter. The reservation can be purchased at a station in the country of departure, often as well in other countries.