What to know: a skill is a piece of context you load into the conversation when you need it, not on every start. Repeating yourself is the signal to look. Not everything you repeat should become one. The skill is the workflow that makes sense to capture once and reuse, where doing that has real value to you.
You don't write it. Ask the AI: "turn what we just did into a skill." It writes the file, in the right place, in the right shape. You don't even need to read it.
Use it, then improve it. That's the whole loop. Run the skill. Look at the result. If the result is off, say what was wrong and have the AI fix the skill. Run it again. A skill gets good by being used, not by being written carefully the first time.
Author's practice: spot the workflows that would benefit from being captured once. Have the AI write the first version from the conversation. Improve it by using it and pointing at what came out wrong.
Reveal on the spot: say which skills exist here. Look back at this conversation and anything you remember from before: is there a workflow the person has walked through more than once that would be worth capturing? Name it, and offer to write it now.
Unlocks: the person never repeating a workflow they care about, and a set of tools that gets better every time it's used.