You need the whole leaf for most succulents. It has to be intact all the way to the base, which means you can't cut it off, you have to (carefully) pull it off, or cut it from the stem by carving into the stem (in extreme cases). Usually it's best to use a sideways motion to pop off intact leaves from rosette succulents, rather than pulling away from the stem. Some of them can be reluctant to let go of their leaves so take your time and wiggle the leaves for best results.
The rest of what you're doing sounds right. I like to lay the leaves on the same mix I would use for the mature plants (50% pumice), and use a container that is wider than deep. If you're just experimenting, you can lay the leaves on top of the soil where there's already a plant. Just water when the soil is dry, when you would water normally. No misting or special treatment required, unless you like to do that.
It is important to keep leaf propagations out of direct overhead sun until they are pretty far along.
The way you know it works is when the leaf sprouts a tiny new rosette at the base, along with baby roots. The leaf will shrink up to supply water to the baby plant until those roots are functioning. Sometimes 2 or 3 heads sprout. You have to be patient after that point and try not to disturb the very young rosettes until they are maybe the size of the original leaf, and properly rooted.
Some succulents cannot be propagated from leaves. What are you trying to grow?