If there is no AppImage of your favorite application available, please request it from the author(s) of the application, e.g., as a feature request in the issue tracker of the application. For example, if you would like to see an AppImage of Mozilla Firefox, then please leave a comment at https://bugzilla.mozilla.org/show_bug.cgi?id=1249971. The more people request an AppImage from the upstream authors, the more likely is that an AppImage will be provided.
If you are an application author yourself and would like to provide an AppImage of your application, please start at https://github.com/probonopd/AppImageKit/wiki/Creating-AppImages.
Essentially, there are multiple options depending on your situation:
Convert existing binary packages. This option might be the easiest if you already have up-to-date packages in place, ideally a ppa for trusty or earlier or a debian repository for oldstable. In this case, you can write a small
.yml file and in many cases are done with the package to AppImage conversion. See examples OR
Bundle your Travis CI builds as AppImages. This option might be the easiest if you already have continuous builds on Travis CI in place. In this case, you can write a small scriptfile and in many cases are done with the AppImage generation. See examples OR
Run linuxdeployqt on your Qt application. This option might be the easiest if you have a Qt-based application. In this case, you are probably already using
macdeployqt and now can use
linuxdeployqt in the same fashion with the
-appimage argument and in many cases are done with the AppImage generation See example OR
Use electron-builder. This option might be the easiest if you have an Electron-based application. In this case, you define AppImage as a target for Linux (default in the latest version of electron-builder) and in many cases are done with the AppImage generation. See examples OR
Manually create an AppDir and turn it into an AppImage.
AppImageExtract have been replaced by
appimagetool. You can find it at https://github.com/probonopd/AppImageKit.
Please see https://github.com/probonopd/AppImageKit/wiki for FAQ.