Reception of other stimuli goes down slowly continuously throughout.
Breathing becomes deeper.
vocalisations may start.  These will increase in volume throughout, until otherwise noted.
breathing begins to have twitches.
Breathing and hearing structures oscillate slightly.
body starts shuddering slightly.
eyes may open and close slowly.
muscles in extremities may relax and contract slowly, mainly in the digits.
Muscles in the legs and tail relax and contract more.
Cognition just begins to fade.
These factors increase.
Voluntary motion begins to disappear.
The body may heave itself slightly.
Vocalisations begin to die down slightly.
Breathing is now fully irregular, and vocalisations are mostly brief, accompanying almost all in/exhalations.
Voluntary motion is almost gone.
Factors decrease, aside from loss of voluntary motion, which increases, and lowering of reception of stimuli, which simply continues.
Voluntary motion is gone.
The eyelids close themselves.
Cognition is almost gone.
Breathing and hearing structures relax.
Cognition probably stops.
The eyelids begin to relax.
The twitching stops altogether.
The last thing that remains is the motion of the eyelids.
All motion stops and the subject is unresponsive.  Breathing and heartbeat are normal, the eyes are about half-shut, and the mough hangs open.

Going back down, this all goes in reverse.