At German train stations
You can make reservations at a ticket office in the larger train stations in Germany. Tell the staff that you have a German Rail Pass and that you only need a seat reservation. You can usually make reservations up to 3 months in advance.
Online through the German railways website
You can make reservations online through the DB Bahn train timetable. Enter your journey details in the timetable and click on Search to see the available trains. Next, click on the link Reservation and select your seat(s).
You can use the following credit cards for payment: American Express, Diners Club, JCB, Mastercard and Visa Card. You will receive your reservation as an e-ticket. You may be asked by the train staff to show the e-ticket together with the credit card that was used for booking.
By phone through a booking center
You can make your reservations through the call centre of the German railways UK booking center. They have English speaking staff and they can book all seat reservations in Germany.
You can contact the booking center through this booking form (mention that you have a German Rail Pass) or by phone: (+49) 180 6 99 66 33 (Monday to Friday 9 a.m. to 8 p.m., Saturday and Sunday 9 a.m. to 1 p.m. CET).