Jill's way (Lauterbrunnen to Kleine Scheidegg to Grindelwald) works fine and certainly sounds more scenic, but note that it is not fully covered by a Swiss Travel Pass, while the Lauterbrunnen to Zweilutschinnen to Grindelwald way is fully covered. Since I have a Swiss Travel Pass, that's the way I went. However, other passes have other coverage, so depending on what you're getting, it may not be any more expensive to go via other routes.
"But I haven't figured out how to add the trip I booked to the app, which one can do"
I'm using the mobile SBB website on a Samsung phone (saves space compared to downloading the app), so my advice may not work on other configurations. But after I put in my origin, destination, and date and time of travel, I get all the travel options. If I choose one, it then expands to show all the steps (number of stops, platforms each train or tram or any other conveyance will be arriving and departing from, walking required, etc). Scrolling below that, I get the option to "Enter in the calendar." If I choose this, it downloads a small file, and I then click it. This gives me all the details, and I can choose to put it in my phone's calendar.
The big advantage of this is, once I've put it in the calendar, I no longer need a data connection to use the information. I'm roaming with T-Mobile US here in Switzerland, so my data is free but SLOW. Therefore, what I've taken to doing is, the night before when I'm on the hotel's Wi-Fi, I look at various possible trips, and put them in my phone's calander (it warns me that I have conflicts, as I usually have input lots of options). Then on the travel day, I choose the one I need, follow it, and delete the unused ones.
Also, you can put in an address, and the SBB website will give you ALL the door to door details. For instance, I didn't just put in Lautebrunnen to Bern, but I put in my Bern hotel's address; SBB then told me which tram to take from the Bern train station, and even priced the whole thing as one journey (handy if you don't have a transit pass and want to know costs).