In Mixamo’s auto rigger panel, the symmetry line is not centered, but a bit off to the left. The blaster (MGM Mega-Buster, if we're being technical) on his left arm was smaller than his right hand. Even though the Megaman character I modeled was centered perfectly, he wasn’t 100% symmetrical. Make sure your character's position is set to X - 0, Y - 0, Z - 0 before exporting out to FBX for Mixamo. Be sure the material is set to Projection/UVW Mapping.

Open up the material editor and apply the baked texture onto all the channels you baked out. Now all that is left to do is create a New Default Material.

If normal and bump maps are ticked, be sure to also tick the Evaluate Bump checkbox.

If you have bump channels, luminance channels, etc, be sure to also have these channels' checkboxes ticked.

So under the Options tab, be sure to tick Use Polygon Selection.

Since we baked down our model, we have a bunch of selection tags and multiple texture tags.

If you do see seams in your final render, increase to 2px)

Pixel Border set to 1 (adds 1px padding on your UV to avoid seams in your mesh.

Supersampling set to 0 (if you're using shadow, you may want to increase this number).

Set the resolution Width & Height somewhere around 1024x1024 to 2048x2048 (depends on resolution you require and how close your camera is zoomed in on your character).

Under Bake Material, add the name of your texture in the Filename field and choose with the (…) button where you want that texture to be saved.

Select the object and go to Tags/Material Tags/Bake Material.

Yeah, it's not that kind of baking, but this is why you shouldn't edit while hungry

Mixamo blender