Tiling a shower is harder than it seems. Sal Diblasi and I have some very detailed instructional videos, but we are professionals and have done this for a very long time.
Most DIY's will struggle with showers, as they require very precise planning and details. The waterproofing is critical, and if anything is not structural sound, failures can occur.
Getting walls plumb and flat is no easy task, and either requires wet shimming the wall board or floating the walls with a mortar bed. Trying to flatten wall with thinset as you set tile is usually an uphill battle that a DIY'er will lose. Leveling clips or spacers will not help in this instance. Leveling systems still require the substrate to be flat.
Waterproofing is also difficult. In this instance the homeowner used Kerdi membrane which requires thinsetting sheets together which can lead to areas of build out, or areas that bubble from the sheets not laying completely flat.
One of the biggest mistakes this DIY'er made was using 1/2'" kerdiboard for the top of the curb, which easily compresses and would have led to a failure if weight was repeatedly put on the soft areas of the curb.
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For the follow up video on the repair:    • DIY RESCUE! Italian Marble Shower  

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