1. Open New Reaper Project
2. Insert Your video
3. View Media Window
4. Add Markers
Play the video start to finish & note down the required sounds. Use m shortcut to add markers on your project.
5. Research for Audio Samples
Search or record the required sound effects relevant to your marker notes. You can refer to Sound Sample Library sources or record your own via your sound recorder.
You can add musical tracks as well. Keep in mind to use royalty free sound tracks.
Use key combinations to speed up your design process. Refer to the Key Shortcuts.