Head over to Deutsche Bahn's site
(or use the DB app if you're out and about as Dan said) and you can plan a journey from any location
within Europe. For example, I can plan a trip from my hometown right through to the front entrance of Europa-Park:
As you can see, you've even got platform numbers where they're already booked in, and transfer times can be amended too. The downside is you can't book the whole journey with Deutsche Bahn online (only the German legs), but the above timetable will help you make your bookings. Likewise you can't book the whole journey on Eurostar's website, so they have to be done separately.
In addition, different train companies put their fares on sale at different times. German fares go on sale 6 months ahead, French 4 months ahead. Eurostar's fares can go on sale about 11 months ahead. So while you could book a journey on Eurostar's website from St Pancras through to a French station or some main German stations like Cologne or Frankfurt, they'll only be available once their tickets are on sale - when the cheapest Eurostar fares have gone.
The way I did it this year is to book as soon as tickets went on sale for each operator, you can usually set alerts on operator's websites to get a reminder. I was travelling from London>Paris>Europa-Park>Phantasialand>Paris>London. So I booked the Eurostar first along with my German fares, then booked my TGV train and Thalys train (Belgium's high speed operator who does a Cologne>Paris train) later. In terms of booking, I'd recommend following websites:
Any UK website of your preference. I used LNER for the rewards points on my route
Their official website
France: SNCF Connect
, the official English website (You can also book Thalys trains on this site)
A note on Eurostar trains. In an ideal world for my trip, I'd have probably travelled down into Paris, done Europa-Park, moved onto Phantasialand then got the Eurostar back from Brussels Midi. However, Eurostar's stupid booking system wouldn't have classed this as a return - you must leave from the same station you arrived at. Eurostar's return fares are far
cheaper than a one way fare, hence the trip back to Paris to head back to London. Hopefully this will change as Thalys has now merged with Eurostar, so their journey options on the continent with the same operator are far more varied. But as it stands that's how things are.
I'd also really recommend booking first class for French and German trains where the jump in price isn't too much, and Comfort Plus on Thalys where possible. We paid 6-8 euros for each leg on the French and German trains, for Thalys an extra €15. You don't get full service or anything, but the larger seats and as a result quieter cabins are far more comfortable so well worth it!