Cast Beacon of Tomorrows by paying ({6} magic symbol) ({U} magic symbol) ({U} magic symbol) .
Riku's first ability triggers - choose not to pay.
Holding priority, cast Narset's Reversal by paying ({U} magic symbol) ({U} magic symbol) , targeting Beacon of Tomorrows.
Riku's first ability triggers, causing you to pay ({U} magic symbol) ({R} magic symbol) to copy Narset's Reversal, with the copy targeting the original Narset's Reversal.
Resolve the copy of Narset's Reversal, copying the original Narset's Reversal targeting Beacon of Tomorrows, and returning the original Narset's Reversal to your hand.
Resolve the copy of Narset's Reversal created in step 5, copying Beacon of Tomorrows and returning the original Beacon of Tomorrows to your hand.
Resolve Beacon of Tomorrows, causing you to take an extra turn after this one.
Repeat each turn.
Notes
You may pay an additional ({U} magic symbol) ({R} magic symbol) to copy the extra turn spell, and only need to do this loop during your second extra turn.