Turn off find my iPhone

I recently used my cell phone insurance to replace my iPhone XR. Yesterday I mailed out the old iPhone after receiving my new one. The return Instructions clearly state that the “find my iPhone “ feature MUST be turned off (on the old one I returned). I know it can be done at iCloud.com, but how??

Can someone provide me instructions on how to do this? Like what to click on, etc. Thanks much and Happy New Year.