I played around with these in the fall. I have not used them much since.
This is a fairly new way to customise Unity and has lots of potential. I
think we are learning as we go. It is basic scripting. You use a text
editor like gedit or kate. The procedure and syntax are fairly uniform, so
pay attention to them. You can cut and paste and modify the contents of one
and save it to a new name. The line that says Exec= is the actual launching
command. You can learn them by finding the actual application launcher name
from the repos or from Synaptic or from looking at the properties of a file
or launcher in the Dash. Note how spaces are not used and used and how
switches are used after the launcher. You may have to launch a terminal and
type the launcher command followed by --help to get a list of the switches
and what they mean. For example: nautilus --help or nautilus -h

I don't think there is much of a guide for this, yet. People who do a lot
of this are helping those like you who want to learn. I expect that this
will evolve into something like what it happening with GNOME Shell where
eventually these will be stored in a specific site and people can download
and install them quite easily. It is just cut and paste.