Add Progress Bar with Background Thread

I have a long-running task in my [app] that [processes files / converts data / downloads multiple items]. I want to show real-time progress to the user.

Set up a background thread that:
- Runs the task without freezing the UI
- Reports progress back to the main thread (percentage or step count)
- Updates a progress bar smoothly as the task advances
- Allows the user to cancel the task mid-way
- Shows "Done!" when complete and re-enables the start button

My stack: [your language and framework here]

I'm learning, so explain each part simply.