Make sure you are using Blender 4.5 or newer versions.

Install it like any other addon. (You can just drag&drop the zip file into Blender)

After installing you will get this panel on the right N-panel:

Press Setup Auto Blend button. This will make the required changes to the scene and creates the compositing setup:

After setting up you will get this interface:

Now, select your objects and Press  button to add blending to your objects.

Note: It assigns the materials of the selected objects, not the objects themselves. So any object that uses these materials will be blendable too.

Note: Make sure that the object has a material in order to be blendable.

Note: Blender's Cryptomatte works with materials by their names. So, whenever you change their names you need to reassign them.

Settings:

Blend Distance - blend distance in world space.

Max Size - max blend distance relative to the screen. 1% means the blend distance wont be bigger than 1% of the screen. (Bigger values = Slower to calculate)

Thickness Scale - bigger values cause more blending based on the depth info. Play with this value until you get the desired result.

Mirror factor - how much the edges will be mirrored. 2 means 1:1 mirroring. In most cases you don't need to modify this.

Hardness - higher values result sharper edges. Good for stylised rendering

Hardness Midlevel - extra control for adjusting threshold of hardness

Mask:

You can press one of the buttons to automatically setup a mask.

Alternatively, you can create a custom mask by connecting your mask to "Mask" input of the AutoBlend node in Compositor.

Feel free to reach me if you have any questions.