It maters if it's MIM or Forged..
The MIM hand spring is loose after the hand is removed & the forged is pinned into place..
There's a thread down a bit on a 642 hand that shows some nice pics that 500 Magnum Nut posted..
I assume your working on a MIM being your asking how to put it back in..
Have a look at this post below.. http://smith-wessonforum.com/s-w-smithing/89800-how-does-hand-spring-fit-442-642-a.html
It'll help you a bunch..
Gary/Hk