Installation

1 - go to Edit -> Preferences then select add-ons tab from the left. 

2 - Click the Install button and locate the add-on zip file, select it and click install add-on.

3 - A new item will appear with a check box near it, enable it and now you are ready to use the add-on.

How to use it

Here is a a video tutorial 

1 - Load your 3D model into blender

2 - It is better to remove unnecessary objects from the scene, you can also delete the  cameras, the add-on will automatically add a configured camera.

3 - Locate the add-on options by opening the right side panel by pressing N key then selecting the Misc tab, you will find  a panel called Sprite 2D .

4 - Select the armature of the model, If the animation doesn't happen in place make sure to click on "Make in place" button . This will make the animation happen in place,  which is necessary for generating proper 2d sprite animation.

 5 - set the Camera angle to the value you like, this angle is relative to the horizon and measured in degrees, for top-down game use 90, for side-scrolller games use 0, and any value between  and 90 for isometric game ( the common value is 45).

6 - set the output resolution, each frame will be rendered as a square and this value will define the  length of the square  side.

7- Click "Setup camera" button, this will add a configured camera to the scene and will auto detect the start and end frame for the animation, it will also set the camera scale value so all the model appear in the camera during the whole animation.

8 - Set the render angles value which defines how many angles the animation will be rendered from, this images will explain the idea more

9 - Adjust the start frame, end frame, and frame step values. Some animations might not require rendering all frames so you can adjust the frame step.

10 - configure the environment and add lights if you like the same way you usually do in Blender.

11 - Select the output directory, and click "Export" button and wait for the render to finish.

12 - Locate the output folder you selected you will see a folder created for each render angle, each folder contain the rendered animation.