Go to the DB itinerary page. Plug in your start (Füssen) destination (Interlaken Ost or West) and other data, then search. You will find itineraries that route you via Schaffhausen (short route through Germany) or Basel. You aren't buying a ticket to Interlaken but this will give you a look at what the whole trip is like.
Based on what you find there about those journeys (duration, changes of train,) you can then dump that page and do a search to either Schaffhausen or Basel. Buy your ticket to one city or the other. There are different routes offered from Füssen to each of these cities from Füssen as well - some longer and more expensive than others, some with more connections than others.
This is a long, nasty train trip all things considered. I wonder if there isn't a way to re-arrange your trip to make the approach to Interlaken easier or to break up the trip with a stop somewhere in between. Füssen is at the very end of a trunk rail line - maybe you drop it and get your alpine experience elsewhere. Füssen is only one of hundreds of nice destinations in Germany, a place international tourists flock to readily, like ants to a picnic, but not necessarily justifiably. If you'll be in Salzburg, nearby Berchtesgaden is more spectacular IMO and less problematic to travel to/from. Lindau and Lake Constance are nice stopover locations.
