How to Create Custom Launch Icon in Ubuntu

[Desktop Entry]
Encoding=UTF-8
Name=Calculator
Name[fr]=Calculatrice
...
Comment=Perform calculations
Comment[fr]=Effectue des calculs compliqués
...
Exec=gcalctool
Icon=accessories-calculator
Terminal=false
Type=Application
StartupNotify=true
Categories=GNOME;GTK;Utility;Calculator

For details see https://help.gnome.org/admin//system-admin-guide/2.32/menustructure-desktopentry.html.en