Start off with what's under the skin with a healthy human/animal. Understanding how the skeletons and muscles work will let you modify them more realistically. I liked to go to the local museum and sketch the skeletons, taxidermy and curiosities there before designing anything when I started drawing trolls, and I would definitely extend this advice to anyone wanting to be likewise creative. This picture was just one of the first results on google when I typed "human skeleton"
How will you change the bone structure? I've extended a few parts of bone which are naturally more protruding, changed the skull shape to make the mouth bigger and changed the angle at which the legs attach to the body which would give this troll a more ape like gait. I'm also making wing like flaps, just because why not
I also like to turn fingers and toes into claws, and elongate teeth because these *are* monsters after all
Brownish and purpe-ish colours for the flesh, and a dirty cream for the bones are what I usually stick by. The bones don't need to show of course, but remember when they do show that it's the muscles which move the body so you can't have entirely bone limbs if you want them to move, at least have a little flesh attached to it ^_^
Draw where you envision the muscles fitting in. Look at the normal muscles picture and imagine how they will need to change to adapt to the new bone structure. This is a super rough estimation, but I find it helps a lot, and it helps me imagine how the troll moves
Some darker colours. Blue/green give it a dead-ish look as mentioned in the previous tutorial and comment, and gives a nice bruised rotten feel to it. This isn't so much shading due to light, but where the bones are rubbing and the flesh and skin is thin and sick. Some darker icky brown to the bones which are visible.
Some veins and arteries in dark reds and blues as well as. Darkest around where the bones are protruding, and generally otherwise around where they'd naturally appear on a person
.... I am probably going to regret posting this very soon........