Cast Galvanic Iteration by paying , copying the next instant or sorcery spell you cast.
Cast Reality Spasm by paying at least , targeting at least three permanents and choosing the untap mode.
Galvanic Iteration triggers, copying Reality Spasm, targeting at least three permanents.
Holding priority, activate at least two mana-producing permanents you control by tapping them, adding at least .
Activate Errant by paying and tapping it, copying the Reality Spasm copy, targeting Errant and at least two mana-producing permanents you control.
Resolve the copy of Reality Spasm from step 3, untapping Errant and at least two mana-producing permanents you control.
Repeat from step 4.

It is recommended to pay at least extra when casting Reality Spasm initially. If you choose not to, you will not get infinite untap of permanents or mana from those permanents, unless one of the mana-producing permanents taps to produce two or more mana.