"Before we demonstrate exactly how you can accomplish that, we
should clarify a few things about the remote computer. First off,
its operating system should be either Linux, BSD or Mac OS. The box
must be online and capable of accepting inbound network
connections, that is to say connections from the Internet. What the
latter usually implies is that the proper port forwarding or NAT
rule is already setup in the router the computer connects to. More
specifically, the remote computer runs an SSH server and accepts
inbound connections to the appropriate port (the default is 22).
Furthermore, the firewall it may have enabled should allow for
connections to that port.
"As for the client machine, i.e., the laptop or even the desktop
computer you intent to establish a secure tunnel from, it may be
running any operating system you like."