I choose to travel from Paris to Bern and then stay one night in Bern and the other day I go to Luzern.
I choose to do this because I arrive at 10pm in Bern and I want to arrive in day to Luzern, I don't regret this because I like a lot Bern even I was for only some hours.
The TGV took about 5 hours from Paris to Bern and then from Bern to Luzern it is about one hour time.
I buy the tickets in the website and choose first class, if you are traveling with 4 people choose the club four option that way you are going with all your family in a 4 seating position seated in a group. In the tgv online website you can select your seat.
Also I buy the train ticket from Bern to Luzern in the SBB.ch website, and after I do that I can't view the time I choose in the ticket because you could take any time the train, it goes every hour from bern to luzern.
The tgv trains are cheap compared to the swiss trains that are very expensive.
The tgv tickets I buy online were sent to me via email, then I print them and that was all very easy.