Carving a Foot Ring without Poking through the Bottom
Okay. Your bowl is centered upside down on the wheel, and you've got a trimming tool in your hand. This is the part people either get real careful about or real reckless about, and both ways get you a hole in the bottom of your bowl.
The foot ring is just a raised ring on the bottom that lifts the bowl up off the table a little. That's it. It's not decorative, or not only decorative. It keeps the bowl from sitting flat on its whole underside, which matters for how it looks and for how it sits once it's glazed. A bowl with no foot ring looks like it melted. A bowl with a good one looks like somebody made it on purpose.
Finding where to cut
Start the wheel spinning slow. Take your trimming tool and rest it lightly against the clay near the outer edge of the bottom, not right at the edge, maybe half an inch in. You're going to cut a shallow groove there first, just to mark where the outside of your foot ring will be. Don't go deep. This is a line, not a trench.
Now find the inside line. Picture a ring about as wide as two fingers, sitting between that outside mark and the center. That ring is what stays. Everything outside it and everything inside it gets trimmed away, down to a certain depth, and the ring is left standing proud.
The part where people panic
Here's the thing that gets beginners. You're trimming from the outside in, working your way toward the middle, and the whole time you're thinking about that ring. But the danger isn't the ring. The danger is the middle. Once you get past the ring and start trimming the center section down, that's where the wall of the bowl is thinnest, because that's where the bottom of your bowl actually is. You already made that decision weeks ago when you were pulling walls and shaping the inside curve. Trimming doesn't create thickness. It only removes what's there.
So go slow in the middle. Take thin passes. Stop and press your thumb gently against the underside every few passes, right where you're cutting, and feel for warmth or thinness. If it's getting flexible or you can feel your thumb pressing close to where your other hand is on the inside, stop. You're close enough.
I trimmed a bowl too thin once and put the tool clean through the bottom. Just a little hole, right in the center, and I sat there looking at it for a while like it might fix itself. It didn't. What I did instead was cut the whole bowl in half so I could actually see the wall thickness I'd been guessing at the whole time. Turned out I'd been trimming way past where I needed to. That's the only way I really learned what "thin enough" looks like from the inside. I'm not saying go ruin one on purpose, but if you do it by accident, at least go look.
A gap I'll admit
Some people can eyeball this perfectly from the first bowl. I couldn't. I still slow way down on the last few passes and I still lose one here and there. At the end of the day it's a guess informed by feel, and feel takes reps.
Checking your work
Once you've got the ring standing and the middle trimmed down evenly, stop the wheel and lift the bowl up to the light, held at an angle so you can see the bottom silhouette. You're looking for a shadow that's roughly even all the way across, no thin spot glowing brighter than the rest. If you see one bright spot, that's a thin spot. Leave it alone at that point. You've done enough.
The kid question
A Primary kid asked me once what I do all day, and I told her I make bowls. She asked why. I didn't have a real answer for her, and I've thought about that more than I expected to for something a seven-year-old said in passing. I still don't have a clean answer. Maybe part of it is this exact moment, lifting a bowl to the light and checking if you did it right. There's something in that. I just can't put words to it very well.
My opinion, for what it's worth: this is the part where people quit. Trimming is slower and less exciting than throwing, and a foot ring feels like a fussy detail nobody will notice. I think finishing the piece is the whole difference between a hobby and just a story about the one time you tried a wheel. Push through this part.
Before next time
Trim all your leather-hard bowls this way, even if the first one or two come out uneven. You'll feel the difference in your hand by bowl four, and that's worth more than getting bowl one perfect.