First, I primed them all with Blood Red spray primer.
Then I base coated all the flesh parts (so not the carapace, scything talons, or various claws) with a pretty serious coat of Blood Red paint, which isn't the same color as the Blood Red spray primer. I apologize for the bad picture. You'll just have to take my word for it. Anyway, you don't have to worry about getting paint in any of the recesses here, since that will be taken care of later anyway. And if you leave a little bit of the darker spray primer showing, it'll be fine.
Next, I drybrushed the flesh areas with Blazing Orange.
I then painted all the recessed areas of the flesh with Scab Red.
Next I did an extreme highlight of Fiery Orange. The highlights will look very extreme at this point. Don't worry, that is normal.
Then covered the flesh areas with Baal Red wash. This will blend in the highlights a little bit, and add a lot of definition to the recessed areas. This also makes it look really shiny, but don't worry about that.
This is probably the most labor-intensive step - painting all the carapace, scything talons, hooves, claws, teeth, and anything else that isn't flesh, with Chaos Black. It may take a couple of thin coats.
I then highlighted all the black parts (except the teeth) with Shadow Grey. I pretty much just did a simple outline of the different areas of carapace or claw, plus a little in any of the little cracks or nicks. Better painters could probably do something that looks a lot better, like that feathering technique that Games Workshop does on all their Tyranid carapaces. My way is pretty quick, and easy to do for someone like me who isn't that good at painting, and looks decent enough. For a lot of it I don't use the tip of the brush, but run the edge of the brush along the edge of what I'm highlighting.
Note that if your carapace is some color other than black, you'd probably want to do more than a two-tone scheme. With black it looks okay. Also, if you want the claws and hooves to look a little different than the carapace, painting them Shadow Grey and then applying a black ink or wash can have decent results, but it doesn't look black enough for what I wanted the carapaces to look like. Which is black. (I mean, how much more black could it be?)
Next, I used a yellowish color with no label to paint the teeth and eyes, leaving a little black outline. The color is probably similar to Desert Yellow. I painted the tongues and the fleshy parts of the Adrenal Glands with Elf Flesh. I also painted the bulbs on the Toxin Sacs with Scorpion Green and highlighted them with Bilious Green.
It is difficult to make out in this picture, but for this step I applied Skull White to the teeth and eyes, making sure to leave some of the yellowish color showing. I also added a dot of Skull White as a glint on the Toxin Sacs, as well as highlighting the Elf Flesh parts of the Adrenal Glands with it.
I applied a pretty watered down wash of Magenta Ink to the fleshy parts of the Toxin Sacs. I also added vertical slits of Chaos Black for eyeballs, as I think it makes them look much more sinister than having blank eyes. The base was painted Goblin Green and flocked. To finish the models, I first apply a very generous coating of Krylon Crystal Clear Varnish. When this is completely dry (after a day), I do a coat of Krylon Matte Finish. The gloss varnish protect the paint job really well, but leaves the model looking very glossy. The Matte Finish removes this glossy look. For the hormagaunts, I tried not to get too much of the Matte Finish on the Carapace, so that it would remain looking a little shiny. At the very end, I painted on some 'Ard Coat finish onto the bulbs of the Toxin Sacs and the fleshy top of the Adrenal Glands to make them look extra slimy.
And that's it. Not too much effort or skill required to get a decent looking result.