Motion Capture

Maya has a great tool for Motion Capture retargeting, which is called humanIK. Kangaroo is simplifying this for you.

If just turn on the function humanIkSkeleton()

it'll create this skeleton:

This function also needs the T Pose. So just pose your character into TPose - just left side is enough:

Unclear about the thumbs?

To see what rotations the thumbs need to have, just open your motion capture and set it to T Pose (either by going to first frame if it's got a t pose, or by zero-ing all rotation values).
On a Skeleton Cache from Mixamo it looks like this:
But in some other examples you might find them oriented more straight to the palms.

And click the button Record T Pose:

Then build again. To verify that it works, turn on the HumanIkVis, and check that it's in TPose:

From here you can use the Motion Capture Tool to connect it to a Skeleton Cache.