In order to implement a Mixamo animation in Unity, you can use the auto-rigging from Unity, if you don't know how to skin animation to the mesh. You'll have to follow the next steps.
Upload your character FBX:
Select an animation:
Click on Download, and do the next settings:
After importing your animation in Unity, you'll see a weird movement, on your character, that's because of the rig, so we'll have to change that.
That's how your rig panel on your animation should look like, Humanoid, and Copy the avatar from the mesh you just uploaded on Mixamo:
Weird animation movement ? Yea, that happens sometimes, in order to fix that you'll have to play with this values around (
I know it's a little vague, but there are no fixed standards, you turn them all on, put them on Original, and then turn off what's wrong):