Unity has a built-in Ragdoll Wizard which lets you easily create a ragdoll of a biped. Go to GameObject -> Create Other -> Ragdoll to open the wizard. Then simply drag and drop the proper joints into the wizard and then hit create. Not all bones need to be assigned. The most important bone that needs to be assigned is the Root. Note that Left/Right Hips are also known as Thigh bones.
- Go here for a more in-depth guide on the Ragdoll Wizard: http://unity3d.com/support/documentation/Components/wizard-RagdollWizard.html
- If you want to create a custom Ragdoll, you will need to create character joints for each ragdolling joint. Go here to find out more: http://answers.unity3d.com/questions/9079/how-do-i-make-a-non-humanoid-ragdoll and http://unity3d.com/support/documentation/Components/class-CharacterJoint.html