Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Call emergency services immediately for any new decerebrate posture.
Tell emergency responders about any recent injury, seizure, illness, medication, alcohol, or possible drug exposure.
Note when the episode began and whether the movements changed, but do not delay calling for help to collect information.
After emergency treatment, routine follow-up may be appropriate for continuing stiffness, movement problems, swallowing difficulty, or rehabilitation needs.
A new episode or reduced awareness remains an emergency.
Do not wait for the stiffness to stop or arrange a routine appointment.
The article also says
While waiting for help:
Check whether the person is breathing normally
Begin CPR if they are not breathing normally and follow dispatcher instructions
Keep the area around them clear to prevent injury
Do not restrain their limbs or place anything in their mouth
Do not give food, drink, or medicine
Avoid moving them after possible head or neck trauma unless safety requires it
Every sign above is quoted from the sofidex article on Decerebrate posture. If you are not sure, err upward — it is always reasonable to be checked.