I gave you a link.
Here's the page:
The important bits are in bold.
But, to paraphrase:
Book the domestic bit via SBB (Baden-Zurich or Baden-Basel).
Book the Zurich/Basel bit on one of the two sites mentioned.
Prices go up everyday (for the bit to Paris) so the earlier the better.
Switzerland to Paris by TGV from €29...
Regular direct TGV-Lyria high-speed trains run from Geneva to Paris (3h10), Lausanne to Paris (3h38), Zurich & Basel to Paris (Zurich to Paris in 4h03). One train a day links Bern and Interlaken with Paris. In Paris, all these trains now arrive at Paris Gare de Lyon.
TGV-Lyria trains have a cafe-bar, power sockets at all seats, and first class fares include snacks & drinks served at your seat, see the TGV-Lyria page for photos & information.
Booking usually opens 120 days ahead and fares start at €29 booked well in advance , rising like air fares to over €100 for a full-flex ticket bought on the day. So book ahead! First class fares start at around €84.
The Geneva-Paris route is particularly scenic... For the first hour the train passes at slow speed through the wonderful mountains of the Haut-Bugey route, crossing the remarkable Cize-Bolozon viaduct over the Ain gorge, before joining the high-speed line for the 2-hour dash to Paris across rural France, see the Paris-Geneva photos here. An increasing number of Geneva-Paris trains are double-deck TGV Duplex - if you book from Geneva to Paris at www.loco2.com or www.trainline.eu you can select an upper deck seat for the best views.
If you just want the TGV-Lyria, as you are travelling from Geneva, Lausanne, Basel or Zurich to Paris, not from further inside Switzerland...
Buy tickets for the TGV-Lyria at either www.loco2.com (in € or £) or www.trainline.eu (in €). Anyone from any country can use these sites, and booking usually opens 120 days ahead. There's no booking fee at these websites and you get a choice of seating options, including upper or lower deck if it's a TGV Duplex or forward-facing seat if it's a single-deck TGV and you're using Loco2 in euros. You print your own ticket.
If you are starting your journey at a station deeper inside Switzerland such as Lucerne, Zermatt, Interlaken, Lauterbrunnen, St Moritz...
OK, so now we're talking a Swiss domestic ticket from your starting point to Basel, Zurich, Lausanne or Geneva plus a TGV-Lyria ticket from there to Paris. The TGV-Lyria ticket is best booked in advance to get the cheapest prices, but the Swiss ticket is usually an 'open' one that can be bought on the day. Swiss trains are very punctual, but I'd allow at least 30 minutes at the interchange station between your Swiss domestic train and the TGV-Lyria to Paris. So here's what you do...
Step 1, book your TGV-Lyria ticket from a suitable 'gateway' station to Paris. Geneva or Lausanne are the best gateway stations for Montreux or Gstaad, Basel is the best gateway station for Interlaken, Lauterbrunnen, Grindelwald, Wengen, Bern or Lucerne. Zurich is the best gateway station for Chur, St Moritz, Davos, Klosters. Obvious if you look at a map!
Book the TGV-Lyria at either www.loco2.com (in € or £) or www.trainline.eu (in €). Anyone from any country can use these sites, and booking usually opens 120 days ahead. There's no booking fee at these websites and you get a choice of seating options, including upper or lower deck if it's a TGV Duplex or forward-facing seat if it's a single-deck TGV and you're using Loco2 in euros. You print your own ticket.
Step 2, then buy a ticket from your Swiss starting station to your chosen gateway station at the Swiss Railways website www.sbb.ch , for English scroll right to the bottom and click 'en'. Tickets go on sale 60 days before travel. SBB's normal flexible Swiss domestic tickets are valid for any train that day not just the one you choose when booking online, no seat reservations are necessary or even possible on most Swiss domestic routes, you just hop on any departure and sit where you like. The price is fixed and availability is unlimited, you can buy one of these tickets at the station on the day if you like, but buying online saves time.
Money-saving tip: Regular flexible Swiss domestic fares are not cheap, indeed the Swiss domestic part of the journey can cost more than the TGV-Lyria journey from Switzerland to Paris! But you can often save up to 50% on Swiss domestic trains by waiting until 30 days or less before your travel date and checking www.sbb.ch for cheap advance-purchase limited-availability Supersaver fares. In the search results a black triangle with a '%' symbol will appear against each departure on which a Supersaver fare available. Supersaver fares are only valid on the specific train you book, no refunds, no changes to travel plans, but they save up to 50% off the normal flexible fare. No cards or passes are necessary to use a Supersaver fare.
Alternatively, anyone from any country can buy tickets from any town or city in Switzerland to Paris at the Swiss Railways site www.sbb.ch , combining Swiss domestic trains with the TGV-Lyria to Paris. You print your own tickets (as the Swiss can e-ticket both their own domestic trains and TGV-Lyria) or you can collect tickets at any main Swiss station.