The signal shows a steady red left-turn arrow while you are turning from a two-way street onto another two-way street. What does the manual say you must do?

A steady red arrow forbids the turn outright, not just the protection from oncoming traffic. The manual reserves the yield-and-turn allowance for a flashing yellow arrow or for a green arrow that has changed into a green light. This answer carries the yield-on-green habit from a solid green ball or an unprotected left over to a red arrow, treating the arrow as one more permissive signal that only asks you to yield.

A steady red arrow requires a full stop, not a rolling slow-and-check. Slowing, looking, and proceeding with care is the flashing yellow light rule, not the steady red arrow rule. This answer applies the flashing yellow light's slow-down-look-and-proceed-with-care habit to a signal the manual says requires a full stop before any turn.

A steady red arrow and a flashing yellow arrow are separate signals with different rules. One requires a stop; the other allows a cautious turn after yielding to oncoming traffic and pedestrians. This answer confuses the steady red arrow with the flashing yellow arrow described in the next panel of the same page, on the driver's-ed shortcut that every left-turn arrow eventually lets you go.