Download Tor Browser only from the official Tor Project website. Any other download page is either an unverifiable mirror or a modified build with something added that you do not want, so if you land anywhere else, close it.
Install it the way you would any application for your system. Do not add extensions and do not resize the window later, because both make your browser fingerprint unique, which is the opposite of what Tor is for. Use it exactly as it ships.
When it opens and connects to the Tor network, you are most of the way through this step. Two small things remain: verify the download and set the security level.