How do you Add Colliders and Rigidbodies to Characters and Have them Be Planted Perfectly??

i'm noticiting i can't seem to plant my characters perfectly on the ground, whether NPCs or My own Character controller.  I add to my NPCS or Character controller a Rigidbody and a Capsule collider but i really dont know what setting to put so that the feet look exactly planted on the ground. it's really diffficult setting the correect Height  and Center Y for the collider to be perfectly on the bottoms of the feet so that the character's feet will seem like they touch the ground.  Anybody got any tips?