This section talks about how to make new cards. It doesn't tell you how to draw the specific images you want on the cards, though. You probably want to use the latex 'picture' environment for that. (Alternatively, if having already looked at some of the existing card style files you think you understand how they use TeX to format themselves, you could give that a shot; this is probably best done by copying one of them and then experimentally changing things a bit at a time.)
(If you want something that looks like memory packets and ingestibles, see those styles for examples of using mechanics-packet.sty, whose internal documentation you should look at.)
How to make a new type of card:
mycard.sty
in LaTeX/Styles/;
it should start out like the existing card styles:
\ProvidesPackage{mycard}
\RequirePackage{extractable} % if mycard is an extractable
\RequirePackage[options]{cards}
mycard.sty
which:
\makecard{front side}
(if it's single-sided or mirror-sided) \makecard{front side}{back side}
(if it's double-sided) To use your new style in a .tex file, just put
\usepackage{mycard}
after the \documentclass{game}
in the preamble. To use it in another style file, just
\RequirePackage{mycard}
there.