Write your first skill
Start from something you already correct by hand.
The problem
Most first skills are too vague. "Write in a professional tone" gives the agent nothing it can check.
What to write
Pick one job you have corrected at least three times. Write down the inputs, the steps in order and the checks you make yourself before accepting the result.
How to test it
Run it on a real task in a new session. Whenever you correct the agent, turn that correction into a step or a check. Paste your draft into the SKILL.md checker to catch structural problems.
Summary
A good skill records your corrections. The acceptance checks matter more than the wording of the instructions.