Follow these steps to import A SINGLE Mixamo's .fbx format character and actions into Blender:
1. Be sure you have the Godot Game Tools add-on installed and enabled in Blender.
2. Choose FILE > Import to bring in your character's .fbx previously downloaded from Mixamo.
3. While in the 3D viewport in Layout View and Object Mode, hit 'N' on the keyboard to bring up the properties panel.
4. From the Godot Game Tools panel on the right-hand side, choose your target as the character's armature.
5. Then select all the .fbx actions previously downloaded by clicking on "join Mixamo Animations". All animations in that folder will be imported. Be patient! Blender needs a minute or two to complete this step.
Follow these steps to import MULTIPLE Mixamo .fbx format characters and actions into Blender:
1. Download each character/actions into a separate folder and add the character's name to each action.The first action should be a t-pose.
2. Create a Blender file and import the first character as for a Single Character above. This first armature should be in the t-pose.
3. Create a second Blender file to import the second character as you did with the first one. Again. The first armature should be in the t-pose.
4. Import the second character into the first Blender file using the t-pose.
5. While that character's armature is selected, click FILE > APPEND.
6. Choose the Blender file for the second character and then choose the folder with the actions. Append all actions in that folder.