JUST BLEED GOD
A little info about this:
The Daemonculaba process began by rounding up some human females as slaves. Once corralled, they were shackled naked within iron cages and force-fed nutrients causing their bodies to widen and bloat to unfathomably grotesque proportions. None of this probably would have even been possible were they not in the Eye of Terror. Next, absolutely fucking mad scientists called Savage Morticians
Left to gestate like a cocoon, the candidate's body was slowly infused with the gene seed and corrupted by the powers of Chaos. Some days later, the candidate is reborn -- without any skin. Provided they weren't already dead, the candidate was then inspected to see if he had the physical match and worth to be a Chaos Space Marine.
However, if the candidate passed the inspection, skin would be cross-stitched onto its body and its new life as a Chaos Space Marine would begin. The skin was harvested from the flayed bodies of human slaves whose flesh had been painfully stretched to vast proportions before being removed while they were still alive to accommodate the demands for large amounts of skin.
While this system mostly utilized loyalist gene-seed (probably exclusively from Imperial Fists stock), it could also use genetic material extracted from "recycled" Iron Warrior corpses, which would then be implanted within the Daemonculaba.
As for the Daemonculaba themselves, if they happened to survive the birthing process, the cycle would days later begin anew, with them again being used as surrogate birthing-wombs...