The instructions you have sound like setting up a local ImageFile to Disc clone operation. Make bootable media (just plain MSDOS floppy, or make that external hard drive bootable itself directly.) Put a copy of GHOST.EXE and the imagefile.gho onto that external drive. Boot off that external drive (set the BIOS). Once booted off the external drive, run Ghost, select Local and Image File to Disc copy/clone and that's it. This much really has nothing to do with the console or any agent installed.
If you don't want to lug around the external HD to each system one at a time, you could setup a basic DOS floppy that has enough networking ability to connect to a network share or a Ghostcast Server. Use the Ghost Boot Wizard, make 20 floppies, stick a floppy into all 20 systems, and then cast to them all.
If this will be a regular occurence, I think setting up PXE is best.
If you already have a console and are familiar with using that console, you could go that way too. If all the machines are bootable already (preinstalled with Windows) and all can be joined to a domain, then you can just Remote Install Client onto each of the 20 systems. Then you can use the console to push the image out to each system (you already have the image you said.)
Good luck,
PH