I'm in need of easy to understand step-by-step instructions on creating an entire song (Introduction, Melody, Refrain, Bridges, and Ending) on a Roland XP-80. I'm a non-professional in need.

Read the manual.

Or you can buy the XP-80 Video Owner's Manual which explains things a bit more clearly. However, entering Performance mode, selecting a preset Key Mode: SINGLE performance (like PR-A:01), then entering Sequencer mode, and then using the manual should do the trick.