Sign Up without Pro
When you first visit Stadia, you are most likely greeted by this page:
The only option at this point is to start using Stadia together with a (free) 1 month trial of "Stadia Pro", which requires adding a payment method to your account in the process.
You can however skip the trial by simply starting a game that is entirely free to play. The following games can be used for this:
Opening a free to play game will greet you with a page dedicated to said game and the option to play it on Stadia - clicking the "Play on Stadia" link will start a login process in which you login via the Google account of your choice.
After logging in you and starting the game, your account is considered a "Stadia Base" (the entirely free tier of Stadia) account. You still have access to the "Stadia Pro" trial (which will require payment methods to be added) if you want to start it at a later point, but you can play and test any of the games above, without said information. Games shared via the "Family Sharing" feature are also available.