Firstly, you need to enable Shell access for the user you wish to log-in with, you can do this in the panel by going to Users -> Manage Users, then clicking Edit for the particular user. You will then need an SSH client to actually log-in to the Shell and issue commands. The client you use will depend on the operating system you use, I use PuTTY under Windows.
More information can be found in the wiki article linked below;
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