How do I boot into single user mode?

Press ESC to stop the auto-boot.
When the list of boot devices is presented:

b PX ISL (where X is your root disc)

And at the ISL> prompt:

HP-UX9.x: ISL> hpux -iS disc(;0)/hp-ux
HP-UX-10.x: ISL> hpux -iS disc(;0)/stand/vmunix

The '-iS' are the flags to init which says come up single user. The rest
of the command is what the bootprocess does automatically.