The core operation in both systems is a modular base mesh to swap out or remove parts and a precision collision system. So, first step is to author a Character mesh/clothing/armor etc designed for dismemberment. Once the Meshes are ready, the next step is script/code the logic in the damage system to handle the collision on specific parts (Ray Tracing? Hit-point Shapes attached to bones?), swapping meshes, spawning particles, triggering animations, etc. Good Luck.