These things have to be done for each release:

Prerelease

• backport all the fixes

• run "make regression"

• update all release version strings, especially in the following files:
  • readme.txt
  • os/debian/changelog
  • os/rpm/openttd.spec
  • os/windows/installer/install.nsi
  • os/os2/installer/make_installer.cmd
  • src/script/api/ai_changelog.hpp
  • src/script/api/game_changelog.hpp
• update changelog.txt
  • sort order: Feature, Codechange, Fix then descending revision number
  • format: - (Feature|Codechange|Fix): ([<subsection (like OSX)>])? <message> ([FS#XXX])? (rXXXXX)
  • merge items that are solving a single goal
• update the known-bugs.txt

Release

• Create Tag via GitHub.

Post release

• wait for the farm
• put a news message on the website
• make a forum announcement
• make a tweet @openttdnews
• update the IRC topic
• forwardport/merge the changes made in the prerelease
• for non-RC releases:
  • create a wiki page and update the release history and template
  • update wiki templates: Version
  • Add the new version to bananas minimal/maximum version list
  • poke package maintainers on IRC: blathijs (debian), heffer (fedora)

After creating a branch for a new version

• Apply changes similar to PR#7319 (a67ee025294278f9113a1c8bcb5ee91d9e174f69)
  • Add a new (empty) AI compatibility script in bin/ai/
  • Add the new version to CheckAPIVersion in src/ai/ai_info.cpp + src/script/api/script_info_docs.hpp
  • Add the new version to src/script/api/ai_changelog.hpp
  • Update the version of regression in bin/ai/regression/regression_info.nut
  • Update the installers (debian/changelog, os2/installer/make_installer.cmd, rpm/openttd.spec, windows/installer/install.nsi)
  • Update the version in src/rev.cpp.in
  • Update the version in src/os/windows/ottdres.rc.in
  • Add a note to src/saveload/saveload.h about which savegame version is used in the branch.
• Backport changelog (and debian/changelog) to master branch
• Update wiki pages: VersionBox