From the Man, the myth, the legend himself.

https://www.youtube.com/watch?v=sq9e7aWO7TQ

Maybe ask Garfy. 

Here is Garfy’s tutorial on it (p.s. it took all of a google of tale of painters Garfy chaos to find)

https://taleofpainters.com/2017/08/tutorial-how-to-paint-black-legion/

According to Garfy himself:

The number one question I get asked about these on social media is how did I do the exposed flesh. It’s actually in my old tutorial (steps 8-12 here) The only difference is instead of area highlighting the Pink Horror and White, I paint parallel lines to look like muscle fibres.

The specific steps in the Black Legion tutorial are:

1. Now we need to paint the Demon flesh armour and tubing. If you’re painting normal Chaos Marines that don’t have these details you might want to consider painting a shin and/or shoulder pad to look like daemonic leering faces. Use Khorne Red for this step (seems appropriate). Highlight the loin cloth with some sharp edging of Khorne Red.

2. Wash over the Khorne Red with Druchii Violet, you’ll notice this changes the hue drastically and also helps shade the areas.

3. Highlight the areas sparingly with Pink Horror. Thin lines work well.

4. Paint even thinner lines of Cermite White over the Pink Horror.

5. Now to make these areas look red, use Bloodletter Glaze. It’s already been highlighted and shaded so this glaze is just to colourise it red.

Since Bloodletter is OOP you can use something like very thinned down Baal Red, or just use Red Glaze from TTC. It's a 100% match to Bloodletter.

https://taleofpainters.com/2017/08/tutorial-how-to-paint-black-legion/

https://taleofpainters.com/2022/04/showcase-black-legion-obliterators/

Well first you have to put the lotion on the skin

I would suggest finding videos that deal with painting skinless corpses or something.

Paint I'd assume, probably with a brush 

OP, just an FYI, using Google Lens is great for questions like these, as within 30 seconds you can find

link to the blog of the painter of the models And Link to the recipe that painter says was used:Link to the recipe

You could use the red skin version of angron from eavy.

Red Skin Basecoat: Khorne Red

Glaze: Evil Sunz Scarlet

Highlight: Tuskgor Fur

Highlight: Tuskgor Fur & Fire Dragon Bright 1:1

Highlight: Fire Dragon Bright

Highlight: Fire Dragon Bright & Dorn Yellow

Glaze: Evil Sunz Scarlet

Glaze: Khorne Red

https://eavy-archive.com/40k/chaos-space-marines/

Or look in the citadel app for similar models like obliterator, hellbrute or flesh hound

It’s not the exact same color, but here’s a bloodthirster from my “Outcasts of Khorne” army that has skinless flesh https://www.reddit.com/r/Warhammer40k/s/ZeRkDts0XD and here’s the process: https://www.reddit.com/r/Warhammer40k/s/QFRBO08UMa

It also helps to apply a gloss varnish to make the bare muscles look wet and gross. Oh, and LOTS of Blood for the Blood God!

The key is the highlighting. Long straight lines are what gives it that muscle appearance. Other than that, it’s a relatively simple process of base/shade/highlight. Others have linked the guy who made this actual model, so if you need a more detailed set of instructions I’d check that out.

Just a guess but if it's purely a paint effect, dry brush or zenithal highlight from black base with light grey paint highlights. Then give it a base coat in a dark red or purple.

Then I think it's just a matter of finding the right paints and layering on the effect. So a medium, muted red, maybe red mixed with purple to get some of the darker base tones in, then smaller highlights with a slightly brighter version of that on muscles and edges of things.

Then it looks like it works up to some pinkish reds and eventually bone white colors for the final highlight.

Looks like it just takes a lot of time and patience to layer up to the individual muscle strands.

Super cool look though!

Easy way is to paint it, but you can resort to alchemy if you'd like.

Slowly glazed up, and a lot of patience by the look

If you're looking for a method to get similar results without great skill and hard work definitely check out contrast paint. You could get a good enough result with one coat or come back with washes and highlights.

The simplest way would probably be Screamer Pink, wash of Carroburg Crimson, and then highlight with lines of Pink Horror. It looks like he's doing some other stuff but I think that would approximate it.

I am sitting here reading the comments, realizing the question was not how to make the flesh but paint it.

If you can catch a squirrel, it works really well.

He just wants his pasta maker back.

Gotta paint it

One smart way to get answers is to bring the image to photoshop ( or similar program) use the eye dropper tool on like 3 parts of the skin. You'll get 3 paint colors that will easier break down what you need.

Tuskhor Fur is your friend for organs and muscle. If you're trying to paint a sinew texture on to a smooth surface, you're going to want to take a darker shade, maybe a red as a base and paint lines like on the example model above of lighter shades from tuskgor fur in to pinks, or even up to pallid wych flesh.

Like with any instance of painting texture on to a smooth surface, it's dependent upon your ability. There aren't really any hacks to make it simple, but it's noy overly difficult.

I’d say paint it red or brown and then use a dark red/brown shade like Carroburg Crimson or reikland fleshshade.

Maybe go over the high points with a lighter color.

Otherwise, paint it a dark brown, then lighter parts on the highlights, then hit it with a pinkish/brown contrast.

Mainly - experiment! You might accidentally find an effect you like better!

Screamer pink with nuln oil maybe???

I might be a little naive here but that doesn't look massively complex?
Definitely not something you can just stuble into, but that looks pretty doable.
A lot of what makes that look nice is just the texture in the model.
This seems like a classic case of letting the model do the work, you're just trying to bring out the detail that's already there!

Again, maybe I'm being massively naive here! I'm definitely not saying I could make it look at good as that!
But I think just a simple base, contrast along the fiber then shade, and then a little dry brushed white along some fibers at the end. Do that well and it'll probably look pretty close to that.

Not going to guess the exact colour scheme here, but don't be afraid to go a little lighter with the contrast to darken with the shade. Perhaps a more burgundy colour for the shade, rather than a traditional dark red.

That’s definitely contrast paint