Navigate to https://tailscale.com/download and click on the "Download Tailsscale for Windows".
From the download area of your browser, open the installer fine and complete the installation.
In the start bar's search area, type "cmd" and open the Command Prompt.
Paste in the pre-authorized join command that you were sent. (Don't use the command shown in the guide - that key will not work.)
Once done, attempt to visit your test server's site.
It is recommended that you right click the Tailscale icon from the system tray and click Preferences -> Automatically install updates to keep the Tailscale client software updated.
To disable Tailscale, click the Tailscale icon in your system tray and click the Exit option from the menu. Relaunch Tailscale from the start menu the next time you wish to connect.