Let's say I have my main branch in GitHub. Me and my friend want to make a second branch called testing where we can mess around with other code before merging it to the main branch. How can I add code to this testing branch. For example, lets say there is an HTML page in the testing branch and the main branch. How can I make changes to the one in the testing branch without touching the one in the main?
I saw slightly similar questions like Github push changes to someone else's branch but that did not help me.
I am using windows if that makes a difference.