how do i back up my contacts online?

https://support.google.com/contacts/answer/9423168?hl=en

Lets say I want to use Gmail. How would I implement that? i have a Gmail email account but I don’t see how that helps. Is there a tutorial that shows me how to have my contacts from my Tmobile phone # back up to my Gmail account? Thank you in advance for your help. I had been a Verizon customer and the change is challenging. Thank you. Tom

If you're using Gmail or Apples version the contacts automatically back up .