Mosaic is invoked from the command line with the command xmosaic (think "Mosaic for X"). Online tutorials for Mosaic already exist, so here's how to find them:
Here is what the control area of the Mosaic window looks like:
To get to a tutorial, choose the Help menu, then choose Manual.... This pops up a new window, and takes you to the NCSA Mosaic Documentation.
To return to this document tree, use the Home button, or the Window History window, to avoid pressing Back twenty times.