Google Apps Script for Sheets — automate a recurring task

Write me a Google Apps Script that runs on a Google Sheet.

What the sheet looks like: {{sheet_layout}}
What I want the script to do: {{script_goal}}
When to run it: {{trigger}} (e.g. 'every Mon 9am', 'on edit of column F', 'button I click')
Optional: do I want it to email/Slack/log somewhere on completion? {{notification}}

Return:
1. THE SCRIPT, ready to paste into Apps Script. Use clear function names + brief comments only on non-obvious lines.
2. SETUP STEPS in order: (a) where to paste it, (b) how to set the trigger, (c) what permissions it'll ask for and why.
3. FIRST-RUN TEST: tell me one row of input I should add to the sheet to verify it works before I let it loose.
4. WHERE IT COULD GO WRONG: 1–2 things that'll cause it to fail silently (rate limits, blank rows, etc.) + how to spot it.

AU English. No deep CS jargon — assume I've never written Apps Script.