Holding priority, activate the creature with Dual Casting attached by paying ({R} magic symbol) and tapping it, copying Cabaretti Charm.
Resolve the copy of Cabaretti Charm, creating two 1/1 Citizen creature tokens.
When the Citizen tokens enter the battlefield, Intruder Alarm triggers twice, untapping all creatures.
Activate Phyrexian Altar by sacrificing a Citizen, adding ({R} magic symbol) .
Repeat from step 2.
Once you have an arbitrarily large number of creature tokens, you may instead choose any of Cabaretti Charm's modes to deal infinite damage to any creature(s) and planeswalker(s) and/or get infinitely large creatures until end of turn.