I'm still learning Mac, but here are my draft notes from the thread at this link:
For apps downloaded from the web:
Open dmg file. Mounts a drive. Either:
1. Drag and drop the icon to Applications from the open dmg window. Eject the mounted dmg file or drag it to trash. Most work this way.
2. Some apps like Office use a custom Installer. Open the dmg file, but inside find a pkg file. Double click on it to install the app. Then before ejecting this type of dmg, note carefully how that app will deal with uninstallation in future.
1. Drag and drop apps: Drop the app's icon to the trash.
2. Apps with their own custom installer: will need to find and use their uninstaller. If the app created a folder for itself during installation, you might find it there.
3. Some apps install themselves as Preferences in OSX, like Perian (a good app for video playback). Open its preference pane in Apple Preferences and uninstall from there.
4. Some apps put an uninstall pkg in the dmg file. Drag that to a place where it will be available in future if you need to uninstall the app.
5. Some apps have a custom uninstall routine that will need to be looked for with Google.