Blender 2.8 - How to bake an ambient occlusion map to a second UV texture on a multi material mesh?

Question

I'm trying to bake an ambient occlusion map onto a second set of UVs onto an object with multiple materials on it (per face) for exporting to a game engine.

I have an object with multiple materials on it. I have materials assigned to each face:

This object also has two uv maps, one for the main textures, and one intended for the AO map lightmap pack unwrap.

I don't actually know if I need a separate material for the AO map, but this is the setup of the AO map material:

In the uv editor, I select the AO UV map, and the AO_Image image I've created, then I click bake:

But then I get multiple messages:

No active image found in material slot (0) for object "Level"
No active image found in material "Material" (1) for object "Level"
No active image found in material "Blue" (2) for object "Level"
Circular dependency for image "brick-pattern-1.png" from object "Level"
Circular dependency for image "grass-tiling.jpg" from object "Level"
No active image found in material "Grass" (5) for object "Level"
Circular dependency for image "AO__Map" from object "Level"
Baking map saved to internal image, save it externally or pack it

And then any materials with image get the lightmap bake, which itself seems garbled for some reason:

Again, the intention is to bake a separate AO map onto a second uv map of a multi-texture object for exporting to a game engine. I'm not sure if this is the correct method. But I'm doing something wrong, because Blender falls over when trying to perform this task and I'm unclear why.

Answer 1

Add a new Image Texture node (with a new image for bake) inside each material and keep it selected (active).

To bake into a new UVMap, create one under Data Properties > UV Maps and enable Camera icon.

Then in Render Properties > Bake, change Combine pass to Ambient Oclusion. After image texture is baked you can use it as usual.