I made this because Git's official instruction is incomplete.
https://git-scm.com/book/en/v2/Getting-Started-Installing-Git to ge the install package and install it.
Now you should have github UI ready for you but if are trying to run "git" in CMD, then follow the rest:
Search where git is installed but some people have it in their C:\Users\xxxxxx\AppData\Local\GitHubDesktop\app-1.0.10\resources\app\git\mingw64\bin therefore they can't find it since AppData folder is hidden. To un-hide:
Select the Start button, then select Control Panel > Appearance and Personalization.
Select Folder Options, then select the View tab.
Under Advanced settings, select Show hidden files, folders, and drives, and then select OK.
Once copy the full path and add it to the "PATH" environmental variable. Here are the steps:
Select Start, select Control Panel. double click System, and select the Advanced tab.
Click Environment Variables. ...
Under the System Variable (or New System Variable) window, append the full path to the end of the variable "Path".
Now you should be able to run "git --version". or post your answers below.