• Go to https://www.mixamo.com and sign up
• Click on "Upload Character" button
• Drag and drop the avatar model in fbx format, for example Asian_F_1_Busi.fbx
• Search and select an animation, for example Dying
• Tweak some parameters of the animation if you want
• Click on "Download" button, keep "Format" on "FBX Binary" and "Frames per Second" on "30", for "Skin" select "Without Skin", and for "Keyframe Reduction" select "uniform".
• Save the file, for example Dying.fbx

See the steps in video: https://github.com/networked-aframe/naf-valid-avatars/assets/112249/be7cf088-76b6-4c9d-b303-5efe610605b5

Using the animation

Put the animation in the ANIMATIONS array in the player-info.js file.

  [
    'Dying',
    '/Dying.fbx',
    {
      ignoreBones: ['LeftEye', 'LeftEye_end', 'LeftEye_end_end', 'RightEye', 'RightEye_end', 'RightEye_end_end'],
      positionMultiplier: 0.01,
      positionOffset: 0,
    },
  ],

Most of the animation on mixamo seems to be scaled by 100, so you will need positionMultiplier set to 0.01. If the feet are not on the ground, you can adjust positionOffset: -0.1.

To play the animation:

document.getElementById('rig').setAttribute('player-info', 'state', 'Dying');