Here's a quick breakdown of how I might approach something like that: https://i.imgur.com/KeQEXtM.png

As you can see, it's not entirely in line with the specific techniques demonstrated in the lesson. This is because Drawabox isn't about showing you how to tackle specific situations, but rather to build up an overall understanding of how to construct basic forms in 3D space and how to combine them in order to develop your spatial reasoning skills. Ultimately you'll be able to work far more flexibly than the rigid processes outlined in these lessons, to create more arbitrary forms whilst reinforcing their relationships with one another in 3D space by establishing their major "planes".

The particular mushroom you chose there was definitely vastly more complex, so I'm not at all surprised that it didn't work out. The main thing to look at in my quick demo is that I established a simple structure (a box-like form for the cap), and then I built the variation directly off those edges, always being sure to have those details return to the simpler structure in order to keep them all grounded in this solid base.