There are several ways to install Annotator. Parts of this answer are from the project's README (and parts of the README were updated from this answer).
PPA
You can use the PPA maintained by Panda Jim (who also happens to maintain the GIMP PPA, among many others). Enter the following commands one by one
sudo add-apt-repository ppa:ubuntuhandbook1/annotator
sudo apt update
sudo apt install com.github.phase1geo.annotator
The PPA supports Ubuntu 20.04+.
Flatpak
You can use flatpak to download it from the Elementary appcenter repositories.
flatpak install https://flatpak.elementary.io/repo/appstream/com.github.phase1geo.annotator.flatpakref
Manually install from source
First install the dependencies.
sudo apt install meson valac debhelper gobject-2.0 glib-2.0 libgee-0.8-dev libgranite-dev libxml2-dev libgtk-3-dev libhandy-1-dev
(libhandy for Ubuntu 20.04 is available in this PPA. In later versions of Ubuntu, it is available in the official repositories.)
Then download the source and run the install script.
git clone https://github.com/phase1geo/Annotator.git --depth=1
cd Annotator
./app install
Flatpak (manual install)
Download the source, and manually build and install with flatpak (the software is not yet available on flathub).
git clone https://github.com/phase1geo/Annotator.git --depth=1
cd Annotator
./app flatpak
And you can open the app from menu (or run it with flatpak run com.github.phase1geo.annotator).
