I guess you are referring to the e-ticket Swiss Travel Pass Flex?
You can activate a travel single day through this website: https://www.activateyourpass.com
You will then receive a QR code which you can show to the ticket inspector, together with your physical passport or national ID card. The code is good for travel on the selected day.
Kindly note, the pass must be activated before boarding a public transport service for the first time on the day of travel.
Ps. We do have an English-speaking SBB community, too. See: https://community.sbb.ch/t5/Questions-and-answers/ct-p/forum_en