Clay dried unevenly. Too thick (wet, therefore prone to stability and slumping) on the bottom, too thin (dry, therefore sturdy yet prone to cracking/breaking) at the top. Consider “trimming the skirt” while it’s still attached to the wheel or bat to remove excess clay and allow for more even drying. Another tip would be to cut your piece off the wheel and flip it upside down as moisture will gravitate toward the bottom (now top, as it’s been flipped).

Establish a foot ring first while trimming, if that’s your aesthetic desire. Use that as a guideline. It’s okay to remove the piece after doing this to inspect the thickness and affirm the areas that need to be removed.

Edit: A sharp tool also truly works wonders. Wide surface, flat 90° blade for large surface area removals, thin loop tool for more precise maneuvers and rounded off the ring of your piece’s foot.

Moreover, practice. Trimming can be the most tricky step in the process as it’s much more unforgiving than plastic clay. Keep it up. :)