Steps to follow for each release

Finalise opam code for release

update version in all the opam files and in configure.ac
run make configure to regenerate ./configure [checked by github actions]
update copyright headers
if you're releasing the first final release of a new branch (e.g. 2.2.0): make sure root_version in OpamFile.ml is set to the final release number (e.g. for 2.2.0, root_version should be 2.2). Make sure that opamFormatUpgrade.ml also contains an upgrade function from the previous version (that function will most likely be empty)
run make tests, opam-rt [checked by github actions]
update the CHANGE file: take master_changes.md content to fill it

[ once bump version & changes PRs merged ]

tag the release (git tag -am 2.2.0 2.2.0; git push origin 2.2.0)
/!\ Once the tag pushed, it can be updated [different commit] only in case of severe issue
create a release (or prerelease if intermediate release) draft on github based on your tag (https://github.com/ocaml/opam/releases/new)
Make sure the repository is in the correct state: git switch --detach <tag>
launch docker using the Docker GUI macOS app
generate opam artifacts, using release/release.sh <tag> from a macOS/arm64 machine, it requires to have Docker and QEMU installed (see below device requirements)
generate the signatures using release/sign.sh <tag>
add releases notes (content of master_changes.md) in the release draft
upload everything from release/out/<tag>
finalise the release (publish)

add hashes in install.sh (and check signatures)
publish opam packages in opam-repository (and add flags: avoid-version and available: opam-version >= "2.1.0" to each packages if this is not a stable version)
update versions (and messages, if necessary) in https://github.com/ocaml/opam-repository/blob/master/repo

a blog entry in opam.ocaml.org
a announcement in discuss.ocaml.org
copy the blog entry from opam.ocaml.org for https://github.com/ocaml/ocaml.org/tree/main/data/changelog/opam
announce the release on the OCaml Discord server

Bump the version with a ~dev at the end (e.g. 2.2.0~alpha~dev)
Check if reftests needs an update
Bring the changes to the changelog (CHANGES) from the branch of the release to the master branch