I remember when I was planning my solo backpacking adventure through Europe the most difficult thing for me to figure out before-hand was the train schedule, especially when I wanted to check to see if particular sections of my route were doable or if I would reach a particular destination before dark. For example, in Riomaggiore, Italy, I stayed at Mar-Mar where the reception closed at 5PM. I wanted to be make sure that I would be able to get from Nice, France, to Riomaggiore, Italy, before 5PM.
One resource I found extremely helpful in planning my route and checking train schedules was the Deutsche Bahn website. You can literally type in origin and destination, as well as dates and times if you know them, to see what your journey will look like. You can also see what type of train it is and if you will need to make a reservation for the journey.
When planning your route, I would recommend first making a list of all the regions, sights, tours, and landmarks that you want to see. Write it down on paper and make a complete list. If you are not on a time schedule, the planning your route becomes very simple as you can now just look at a map and plan out a route that will allow you to see your entire list. However, if you only have a certain amount of days, weeks, or months, you may need to trim your list a bit.
Some of the must-sees on my list included hiking in the Swiss Alps, hiking the Cinque Terre, partying in Ios, attending a music festival in Paris and seeing the Eiffel Tower lit up at night, visiting Naples where my family is from, experiencing the Red Light District in Amsterdam, sailing on the Rhine River in Germany, doing a wine tour in Tuscany, seeing the Roman Coliseum, sampling Tapas in Spain, and riding the Ferry from Tarifa to Morocco. Luckily, since these destinations are very spread out, I had unlimited time.
There are a few ways you can go about this. Your first option is to look at your list and choose the top items on your list that you must see. While this may sound like the most logical option, I actually would recommend the second option, where you take a look at a map and figure out which sights on your list are closest in proximity to each other and create a route out of those. If you choose this method, you can always save the second half of your list to create a separate trip that allows you to explore these other regions you missed your first time around. Also, remember to look up any festival/concert/show dates you may want to attend and plan your route accordingly, as experiences like these can really enhance your trip.
I think the most important piece of advice I can give on planning your backpacking trip for Europe is DON’T STRESS. If you miss a train, are late to getting to a certain place, or have to miss something you wanted to see it is not the end of the world. Trust me, you will end up seeing way more than you expected whether you plan every detail or just fly over and wing it (which, by the way, tons of travelers successfully do all of the time). So just enjoy your time abroad and realize that you are having the experience of a lifetime.