Before the release

Write the release notes.
Branch the tree to the appropriate "Integration" branch.

Prep the source tree

Note the release date in the release notes and commit this change: git add CHANGELOG.rst; git commit -m "Release $VERSION"
Tag this commit: git tag -a -m "Release $VERSION" $VERSION -s (drop the -s to avoid GPG-signing the tag.
Push to upstream: git push --tags origin Integration_$VERSION

Create a clean checkout and build the tarball

mkdir ~/fish_built/release; cd ~/fish_built/release
git clone https://github.com/fish-shell/fish-shell/ -b $VERSION fish-$VERSION
cd fish-$VERSION; ./build_tools/make_tarball.sh

You should get the path to the tarball (something like ~/fish_built/fish-$VERSION.tar.xz and a SHA-256 sum.)

Sign the tarball:

gpg --sign --detach --armor fish-$VERSION.tar.xz

Build the Linux source packages.

# Copy `~/fish_built/fish-$VERSION.tar.xz` to the staging area 
cp ~/fish_built/fish-$VERSION.tar.xz ~/fish_built/release/
# Make the Debian staging area
mkdir -p ~/fish_built/release/dpkgs; and cd ~/fish_built/release/dpkgs; and ln -s ../fish-$VERSION.tar.xz fish_$VERSION.orig.tar.xz
# Unpack the tarball
tar xf fish_$VERSION.orig.tar.xz; and cd fish-$VERSION
# Make the vendor tarball
build_tools/make_vendor_tarball.sh
ln -s ~/fish_built/fish-$VERSION-vendor.tar.xz ../fish_$VERSION.orig-cargo-vendor.tar.xz
mkdir cargo-vendor
tar -C cargo-vendor -x -f ../fish_$VERSION.orig-cargo-vendor.tar.xz
# (uupdate is an alternative method if you have a previous version available).
# Copy in the Debian packaging information
cp -r ~/fish_built/release/fish-$VERSION/debian debian
cp -r ~/fish-build/debian/changelog debian/changelog
# Update the Debian changelog for the new version - interactive
dch --newversion "$VERSION-1" --distribution testing

The Debian changelog needs to be inspected.

# Build the package for Debian on OBS (use your GPG key ID)
debuild -S -k3E03B4E97C71CD1C0E94B287FCA50E480C273BBA -d
# Set up an array for the Ubuntu packages
set -g PPA_SERIES (python3 -c "
from launchpadlib.launchpad import Launchpad
launchpad = Launchpad.login_anonymously('fish shell build script', 'production', '~/.cache', version='devel')                  
ubu = launchpad.projects('ubuntu')
print('\n'.join(x['name'] for x in ubu.series.entries if x['supported'] == True and x['name'] not in ('trusty', 'xenial', 'bionic')))   
"
)
# Build the Ubuntu source packages
for series in $PPA_SERIES
    sed -i -e "s/$VERSION-1)/$VERSION-1~$series)/" -e "s/testing/$series/" debian/changelog
    debuild -S -sa -k3E03B4E97C71CD1C0E94B287FCA50E480C273BBA -d
    sed -i -e "s/$VERSION-1~$series)/$VERSION-1)/" -e "s/$series/testing/" debian/changelog
end

Upload the Ubuntu packages

cd ~/fish_built/release/dpkgs
for i in fish_$VERSION-1\~*.changes; dput fish-release-3 $i; end (or other appropriate PPA defined in ~/.dput.cf)

Prep the OBS repository

mkdir ~/fish_built/obs/; cd ~/fish_built/obs;
osc checkout shells:fish:release:3/fish; cd shells:fish:release:3/fish
rm *.tar.* *.dsc
ln -s ~/fish_built/release/dpkgs/fish_$VERSION.orig.tar.xz .
ln -s ~/fish_built/release/dpkgs/fish_$VERSION.orig-cargo-vendor.tar.xz .
ln -s ~/fish_built/release/dpkgs/fish_$VERSION-1.debian.tar.xz .
ln -s ~/fish_built/release/dpkgs/fish_$VERSION-1.dsc .
set RPMVERSION (echo $VERSION |sed -e 's/-/+/' -e 's/-/./g')
sed -e "s/@VERSION@/$VERSION/g" -e "s/@RPMVERSION@/$RPMVERSION/g" < ~/fish_built/release/fish-$VERSION/fish.spec.in > fish.spec

STOP HERE: check the fish.spec file

osc addremove; osc commit -m "New release: $VERSION"

Create the Mac packages

./build_tools/make_pkg.sh
./build_tools/mac_notarize.sh ~/fish_built/fish-$VERSION.pkg <AC_USER>
./build_tools/mac_notarize.sh ~/fish_built/fish-$VERSION.app.zip <AC_USER>

The Mac packages consist of a Mac installer package .pkg and a double-clickable app. Anyone can build these, however both must be code-signed and notarized to launch successfully on users' machines. ridiculous_fish currently holds the code signing certs.

Upload to GitHub

Add the release notes, tarball, tarball signature and the Mac package/installer
For major/minor (X.X.0) releases, add the release notes for the betas (if any) as well. Consider adding the preceding release's notes for point releases.
Point people away from the GitHub generated tarballs with the following text:

Download links:
To download the source code for fish, we suggest the file named "fish-$VERSION.tar.xz". The file downloaded from "Source code (tar.gz)" will not build correctly. The SHA-256 sum of this file is . A GPG signature from <KEY NAME/ID> is available as "fish-$VERSION.tar.xz.asc".

Update the website

Clone down fish-site and follow the "Making a New Release" instructions.

Send a release announcement

Email the fish-users mailing list. The build_tools/list_committers_since.fish script can be used to generate a list of new and returning committers, which helps to recognise the contribution they have made (an idea pinched from the Git release announcements).
Consider cc:ing the various distribution maintainers if you have their address to hand

After the release

There are a handful of maintenance things that can be done at almost any time, but right before the release is definitely the wrong time. Consider:

Running make lint-all
Merge master into coverity-scan-master

Fish - The friendly interactive shell