A skill is a folder of instructions an agent picks up when the job matches. Matt Pocock has put his own set out in the open, and it's worth a read whether or not you ever install it.
What surprised me is what they are for. I'd assumed a skill would be a shortcut, something to get the agent quicker at a job you do a lot. Almost none of these are that.
Brakes
grill-me interviews you about your own plan and keeps going until there's nothing left undecided. tdd makes it write a failing test before it writes anything that could pass. diagnosing-bugs walks it through reproducing the problem and cutting it down to the smallest version before it's allowed to guess at a fix.
His reasoning is in the readme and it's the honest version. An agent that has misunderstood you writes the wrong thing very quickly, and an agent with no way of checking itself writes something that looks right. Neither of those improves with a cleverer prompt. What helps is making it wait.
Getting it
Two ways. As a plugin, which updates itself and can't be edited, or copied into your own project, where you can change it. I'd copy it. Half the value is reading how the instructions are written, and I think you only really take that in by editing them.
The one I want to try here is the debugging loop.