This tutorial was very clear, but when at the end you use the grab tool in the end, I tried to do the same and get that I screw up whatever sharpness I achieve in the previous phase.
This is how my mesh looks after flattening the bottom:
This is how it looks after using the grab tool:
As you can see in the bottom view, the "flat" face in particular gets all bobbly.
Do you have some tips for using the grab tool without distorting the whole result? And does the rest of what I did look alright?
That's just the nature of the grab tool. You can try to mess with the curve settings of the brush where you may get slightly better results, but it will never be able to move your flattened surfaces perfectly.
Alternatively you can hop into edit mode, select the faces of your flattened top, enable proportional editing and make your adjustment that way. It could be potentially slow depending on how many polys are in your mesh, but it would also maintain the exact shape of your selected faces.
As for critiquing your progress so far, it's looking good. It's looking very stylized and kinda gem-like since it's entirely faceted. If you're going for more of a boulder, crags and cracks will help.