find-the-commit.txt
This worked at some point and doesn't now. I have <n> commits between
the last known good version and now.

Symptom: <describe>
How I test whether it's broken: <the exact steps or command>

Walk me through bisecting this with git. Include:
- Finding a known-good commit to start from.
- The exact commands, in order.
- What to do when a commit won't build or run — that's not a
  "broken" result and I don't want to mark it wrong.

Then, once I have the commit, how do I work out which change within it
caused this?

Automate the test if you can. git bisect run with a script that exits non-zero on failure turns twenty manual checks into one command.