How I re-skin monsters

The Black Magga is a creature in Rise of the Runelords adventure path. It is a plesiosaur neck and head on a octopuses body.

Here is the final art in Rise of the Runelords for the Black Magga

What did I use for this?
A Froghemoth

I just changed the tongue attack to a tentacle attack that does the same thing and gave the bite a 10 ft reach. I then described everything as how it looked like the Black Magga did the attacks. simple.