first you need a ragdoll using the enhanced system, a pool of water big enough to drown a ragdoll, and the lifesaver toolset(specifically, bandages and resurrection). once you got those, dip your ragdoll underwater until it drowns, and remove it.(make sure it isnt damaged during this process!) then, enter creative and first remove at least a voxel from each part to modify(make sure to not remove voxels that connect the body parts), including the head regardless of modifying it or not. then, add the changes you wish to make in creative, whether this is modifying the body or adding new parts to said body(make sure to at least add 1 voxel to the head also). feel free to leave creative during this step to rotate to operate on the other side. once done modifying, revive the ragdoll and bandage the head. if done correctly, the blood increase should stop once bandaged, and you will have an uninjured ragdoll! (if it enters a sedated state and doesnt react to any sort of pain, give a quick dip in the pool to reactivate him)

if you wish to also make the new features blend in, you can use this spraycan mod https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=2989485208&searchtext=spray to recolour the parts made in creative, or the current parts(do this only while the ragdoll is drowned)