You can book the whole journey on the Swiss train site (SBB) as there is a direct TGV train from Paris to Zurich. Book a month or two ahead and you should get a good price, as international journeys have special pricing for advance purchase. Note that the Special Offer tickets are either inflexible/non-refundable, or semi-flexible; you have a choice.
Looking today for date in early January, for example, I see an excellent choice departing Paris at 10:22, with changes at Zurich and Chur, arriving in Samedan at 17:45. The full price for this route is 252 CHF, but the Special Offer price is 136 CHF for the non flexible option, and about 10 CHF more for the semi-flexible option.
There are other route and price options, with additional changes along the way. We always prefer fewer changes.
The SBB a website has been “glitchy” the past day or so, so rather than go to the main page at SBB.ch, use this link to the booking page:
https://www.sbb.ch/en/travelcards-and-tickets/tickets-for-switzerland/individual-tickets.html