Blender Rig "specs" for flawless animation re-targeting ?

Is there a tutorial or “specs” sheet for a rig created in Blender in order to be able to re-target existing anims to that that rigged mesh ? (for example, I want to make a custom rigged character in Blender, export it to UE4 and use anims, purchase from Marketplace, with my character without any hassles in UE4)