I start by detailing with brown in technical drawing pens (sadly for me, my favourites, Edding 1800s, are no longer in production - gutted!) then lay down base colours with Tria markers. After that I do the highlighting and colour detailing with colour pencils.
I painted one of these years ago for a bioillustration project, I got very excited when I recognized this guy. Looks amazing! You definitely did it more justice than I did (and made it a whole lot scarier!). When one of your favorite things is random obscure extinct marine predators you don't expect to see them around terribly often, or well illustrated when they are.