How to setup physics
You should setup physics in PmxEditor, because changing values is easier.
But note that you should also test physics in MMD, because physics engine works a little differently.
Adding physics is actually easy, at least compared to rigging or texturing.
Once the bones are complete, select the bones and press Create Rigid Body/Linked Joint.
After that, you can adjust physics to your liking!
credit: Grayちゃん by らりほま & me