Do you have an existing rig? Do you want to use it with Mixamo animations? and then adjust them to your liking? This is the tutorial for you then!
The Basic concepts is you upload your mesh and joints to mixamo and download an animation, then you merge it back in, and hide the mesh that came from mixamo ( you need to keep it). Then you parent the controllers of your mesh to the joints and now you can use your existing rig to adjust the mixamo animation!
https://ace5studios.com/products – check out the free rigs first !
Looking for free rigs to practice animation? Be sure to check out the Shop page!