Inside the file you opened from the download, you should see this:
Spades;fmarkSpadesBody.png
Clubs;fmarkClubsBody.png
Hearts;fmarkHeartsBody.png
Diamonds;fmarkDiamondsBody.png
Joker;fmarkJokerBody.png
Smudge;fmarkSmudgeBody.png
Classic Tabby;fmarkCTabbyBody.png
and so on....
There should be a large group of text under each category such as Feline Body, Head, Tail etc. You need to copy that text, one category at a time.
Using Notepad++ open the markings.cfg file in your Feralheart folder and you should see something similar like this:
[Fbody]
Cheetah;fmarkCheetahBody.png
Cheetah 2;fmarkCheetah2Body.png
Cub Spots;fmarkCubSpotsBody.png
Back Splotch;fmarkBackSplotchBody.png
Jaguar;fmarkJaguarBody.png
Jaguar2;fmarkJaguar2Body.png
Paste the codes at the bottom of the list, in the correct category. So the text in the "Feline Head" part, from the download, goes under the "[Fhead]" in the .cfg file, underneath the current text of course.