Once the download and the installation of the program is complete, enter the activation key when running any application from the Microsoft Office package, for example Microsoft Word. It usually will be the first thing that you will be asked for. Should that dialog not pop up you can enter the registration key in the “File” menu under “Account” (Office 2013/2016/2019) or under “Help” (Office 2010).
Alternatively you can open file menu -> Help. Here you can verify the activation of the program or change the key.