How do I install it?

Linux

Package subtitlecomposer is latest stable version.
Package subtitlecomposer-git is latest git build.

Noble Numbat (24.04) LTS: Repository (amd64) Binary Packages Mantic Minotaur (23.10): Repository (amd64) Binary Packages Jammy Jellyfish (22.04) LTS: Repository (amd64) Binary Packages
Run following in terminal as root: 

                  echo 'deb http://download.opensuse.org/repositories/home:/maxrd2/xUbuntu_24.04/ /' | sudo tee /etc/apt/sources.list.d/home:maxrd2.list

                  curl -fsSL http://download.opensuse.org/repositories/home:maxrd2/xUbuntu_24.04/Release.key | gpg --dearmor | sudo tee /etc/apt/trusted.gpg.d/home_maxrd2.gpg > /dev/null

                  sudo apt update

                  sudo apt install subtitlecomposer
Run following in terminal as root: 

                  echo 'deb http://download.opensuse.org/repositories/home:/maxrd2/xUbuntu_23.10/ /' | sudo tee /etc/apt/sources.list.d/home:maxrd2.list

                  curl -fsSL http://download.opensuse.org/repositories/home:maxrd2/xUbuntu_23.10/Release.key | gpg --dearmor | sudo tee /etc/apt/trusted.gpg.d/home_maxrd2.gpg > /dev/null

                  sudo apt update

                  sudo apt install subtitlecomposer
Run following in terminal as root: 

                  echo 'deb http://download.opensuse.org/repositories/home:/maxrd2/xUbuntu_22.04/ /' | sudo tee /etc/apt/sources.list.d/home:maxrd2.list

                  curl -fsSL http://download.opensuse.org/repositories/home:maxrd2/xUbuntu_22.04/Release.key | gpg --dearmor | sudo tee /etc/apt/trusted.gpg.d/home_maxrd2.gpg > /dev/null

                  sudo apt update

                  sudo apt install subtitlecomposer

Package subtitlecomposer is latest stable version.
Package subtitlecomposer-git is latest git build.

Debian 12: Repository (i586 / amd64 / aarch64) Binary Packages Debian 11: Repository (i586 / amd64 / aarch64) Binary Packages Unstable: Repository (amd64) Binary Packages Testing: Repository (i586 / amd64) Binary Packages
Run following in terminal as root: 

                  echo 'deb http://download.opensuse.org/repositories/home:/maxrd2/Debian_12/ /' | sudo tee /etc/apt/sources.list.d/home:maxrd2.list

                  curl -fsSL http://download.opensuse.org/repositories/home:maxrd2/Debian_12/Release.key | gpg --dearmor | sudo tee /etc/apt/trusted.gpg.d/home_maxrd2.gpg > /dev/null

                  sudo apt update

                  sudo apt install subtitlecomposer
Run following in terminal as root: 

                  echo 'deb http://download.opensuse.org/repositories/home:/maxrd2/Debian_11/ /' | sudo tee /etc/apt/sources.list.d/home:maxrd2.list

                  curl -fsSL http://download.opensuse.org/repositories/home:maxrd2/Debian_11/Release.key | gpg --dearmor | sudo tee /etc/apt/trusted.gpg.d/home_maxrd2.gpg > /dev/null

                  sudo apt update

                  sudo apt install subtitlecomposer
Run following in terminal as root: 

                  echo 'deb http://download.opensuse.org/repositories/home:/maxrd2/Debian_Unstable/ /' | sudo tee /etc/apt/sources.list.d/home:maxrd2.list

                  curl -fsSL http://download.opensuse.org/repositories/home:maxrd2/Debian_Unstable/Release.key | gpg --dearmor | sudo tee /etc/apt/trusted.gpg.d/home_maxrd2.gpg > /dev/null

                  sudo apt update

                  sudo apt install subtitlecomposer
Run following in terminal as root: 

                  echo 'deb http://download.opensuse.org/repositories/home:/maxrd2/Debian_Testing/ /' | sudo tee /etc/apt/sources.list.d/home:maxrd2.list

                  curl -fsSL http://download.opensuse.org/repositories/home:maxrd2/Debian_Testing/Release.key | gpg --dearmor | sudo tee /etc/apt/trusted.gpg.d/home_maxrd2.gpg > /dev/null

                  sudo apt update

                  sudo apt install subtitlecomposer
Add the following to /etc/pacman.conf (need to edit as root): 

                [home_maxrd2_Arch]

                # Subtitle Composer

                SigLevel = Optional TrustAll

                Server = http://downloadcontent.opensuse.org/repositories/home:/maxrd2/Arch/x86_64/
Repository and packages are signed with GPG key DE85E73C17AF00C8E865B04F0073ABF073738FA0.
Update/install key as root: 

                wget -O - http://build.opensuse.org/projects/home:maxrd2/signing_keys/download?kind=gpg | sudo pacman-key --add -

                pacman-key --lsign-key DE85E73C17AF00C8E865B04F0073ABF073738FA0
Finally install the package as root: 
pacman -Sy subtitlecomposer
Or if you prefer latest git build: 
pacman -Sy subtitlecomposer-git

Package subtitlecomposer is latest stable version.
Package subtitlecomposer-git is latest git build.

Fedora 41: Repository (x86_64 / aarch64) Binary Packages Fedora 40: Repository (x86_64 / aarch64) Binary Packages
Run following in terminal as root: 

                  dnf config-manager --add-repo http://download.opensuse.org/repositories/home:maxrd2/Fedora_41/home:maxrd2.repo

                  dnf install subtitlecomposer-git
Run following in terminal as root: 

                  dnf config-manager --add-repo http://download.opensuse.org/repositories/home:maxrd2/Fedora_40/home:maxrd2.repo

                  dnf install subtitlecomposer-git

Package subtitlecomposer is latest stable version.
Package subtitlecomposer-git is latest git build.

Tumbleweed: Repository (i586 / x86_64 / aarch64 / armv7hl) Binary Packages Leap 15.6: Repository (i586 / x86_64 / ppc64le / aarch64) Binary Packages Leap 15.5: Repository (i586 / x86_64 / ppc64le / aarch64) Binary Packages
Run following in terminal as root: 

                  zypper addrepo http://download.opensuse.org/repositories/home:maxrd2/openSUSE_Tumbleweed/home:maxrd2.repo

                  zypper refresh

                  zypper install subtitlecomposer
Run following in terminal as root: 

                  zypper addrepo http://download.opensuse.org/repositories/home:maxrd2/openSUSE_Leap_15.6/home:maxrd2.repo

                  zypper refresh

                  zypper install subtitlecomposer-git
Run following in terminal as root: 

                  zypper addrepo http://download.opensuse.org/repositories/home:maxrd2/openSUSE_Leap_15.5/home:maxrd2.repo

                  zypper refresh

                  zypper install subtitlecomposer-git

AppImage is a self-contained executable with all dependencies included that runs on any Linux system.

It’s quite simple to run AppImages. All you have to do is download them, make them executable and run them.
Here's more details on how to run AppImage.

Download latest git development version: i686 x86_64 aarch64
Download latest release: i686 x86_64 aarch64
Add the Flathub and KDE Apps nightly repositories: 

                flatpak remote-add --user --if-not-exists flathub http://flathub.org/repo/flathub.flatpakrepo

                flatpak remote-add --user --if-not-exists kdeapps --from http://distribute.kde.org/kdeapps.flatpakrepo
Then install the flatpak: 
flatpak install org.kde.subtitlecomposer
Subtitle Composer is already available on most Linux distributions.
You can install it from the KDE Software Center.
Windows Windows build are currently broken.
If you would like to help please let us know. Sources Clone the git repository: 

          git clone http://invent.kde.org/multimedia/subtitlecomposer.git
Instructions for building from sources can be found on the wiki page.