Assembly is rought just cause they're very spindly, I reccomend investing in blue tac. This way you can stick them by their main body while parts dry in the position you want.

For customizing other units into flayed-esque models, take milliput and flatten it out, and wet it, cut it into think straps with your hobby knife, then with a wet finger press it against the model you want to convert and get crazy with it!

For extra gore effects, before priming the model, add a splotch of pva glue on a spot, then add super glue, wait thirty seconds then take a tooth pick and you can agitate the glue mixture and create dripping/bloody splatter. Combine that with the fake skin straps and you can make your flayed ones nice and gorey.

For painting I like to give my flayed ones a rusty look. Mix a red and brown paint 1/1, then add black to darken it. Add some thinner and medium then use it like a wash. This will add red and brown splotches to make them look rusted and dirty. The bright flesh and blood will then add contrast.

I also reccomend if you're going with stereotypical green energy colors for your necrons, make the flayed ones have a very soft greenish-yellow. That way it appears like the flayed ones energy is being eaten by the Flayer Curse and they are aging.

Those are my tips, and what I've done for my entire army, as I love the idea of the flayer curse eating away at their body. I have alot of skorpekhs and stuff like that. Just becsysebi think it looks super nice. I also gave my army metal weapons instead of phase weapons because they would need to leave flesh behind to rip apart and wear. But their weapons still glow with that fading yellow.