Check each clip shows its real size (3840×2160 for 4K). If it shows smaller, iOS shrank it when you picked it.
Pick where in the song your edit begins, like the chorus.
Cuts through all your clips in turn, on a steady beat. Real beat detection comes in Phase 3; for now set the tempo or tap it.
Hold the preview to see the original. Double tap a slider's name to reset it.
Every test render is kept here on this phone only. Nothing is sent anywhere. Copy them to paste to Charlie.