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Making the AppImage executable before running is the required step on Ubuntu. You asked: How to create an AppImage. Below is a concise, beginner-friendly guide and then a short question to check understanding.

How to create a simple AppImage (step-by-step)

Prepare an app directory: create a folder named MyApp.AppDir. Inside it place your application binary in MyApp.AppDir/usr/bin/ and any libraries in MyApp.AppDir/usr/lib/. Add icons to MyApp.AppDir/usr/share/icons/ and a desktop file to MyApp.AppDir/usr/share/applications/myapp.desktop (desktop file must contain at least [Desktop Entry], Name, Exec, Icon, Type=Application).
Make sure it runs: test the app by running the binary inside the AppDir (e.g., ./MyApp.AppDir/usr/bin/myapp).
Download linuxdeployqt or appimagetool: for Qt apps use linuxdeployqt, or use appimagetool for general AppDirs. Get appimagetool from the AppImageKit releases.
Build the AppImage: run ./appimagetool-x86_64.AppImage MyApp.AppDir. This produces MyApp-x86_64.AppImage.
Test the AppImage: make it executable (chmod +x MyApp-x86_64.AppImage) and run it (./MyApp-x86_64.AppImage).
Optional — add update support: include an update information string in the AppImage or use AppImageUpdate-compatible metadata so users can update automatically.

Notes: keep the AppDir layout and desktop file correct. For many common languages/frameworks there are helpers (linuxdeployqt for Qt, linuxdeploy for others) that bundle dependencies automatically.

Quick check — single choice

Which tool directly creates the final .AppImage from an AppDir?

appimagetool
apt
make

Optional: one short sentence about which part you'd like a deeper walkthrough for (AppDir layout, desktop file, or using appimagetool).

Learning: Appimage file on Linux