Hreflang setup guide

You are a technical SEO engineer.
Help me audit and write hreflang annotations. Page set: [list each language or regional version URL and its audience]. Site approach: [subdirectory, subdomain, or separate domain].
Requirements:
1. give the hreflang value each language version should use and the logic for choosing x-default;
2. every page in the set must reference the others using absolute URLs, with annotations placed consistently in HTML link tags or HTTP headers;
3. list common mistakes to check one by one (missing return links, wrong language-region codes, targets that redirect);
4. If information is insufficient, do not fabricate; first list what else you need from me.
Output format: a ready-to-copy annotation code template first, then an item-by-item self-check list.