Okay, I was afraid that this wasn't clear enough... First
1. After step 8 you create another layer and fill it with colour (yes, as you said, with the Paint Bucket). Then add the Crack 2 Pattern to it.
2. Create yet another layer and fill it with colour also. Add the Brick Pattern with Blend Mode "Multiply". Set the visability to 0% (as discribed in step 5).
Now you should be at step 9 / 10.
The connection in Layer 2 is a mistake from my side. It should not be there. Second
It looks like I forgot something in step 13 or 14. After you merged the floor layer, you can activate the other layers again (click on the little eyes). Otherwise they are invisible and the wall is not gonna appear in the background. I guess that was your problem. Sorry!Third
About the Layer Mask. Hmm.. it's hard to explain it like this, it works pretty much like an eraser with the difference that you can change it any time you want. If I have the time I'll make another tutorial just about that. For now you could actually also use the eraser. Choose a very big brush size with a soft border and just delete some parts of the upper floor and the upper wall layer and you should get a similar result.Forth
If in step 32 nothing changes when you make a doubleclick you can just ignore that, it should be okay.
And in case that is not clear enough either:
1. After you merged the wall Layers duplicate that layer.
2. Deactivate the copy by clicking on the little eye and choose the lowest layer you got.
3. Go to the Brightness / Contrast Settings and make that Layer much darker.
4. Activate the copy again.
5. Now you can use the eraser instead of the Layer Mask to delete parts of the wall copy.
Okay, I hope I answered all your questions! If it's still unclear just ask again...