If you don't have one, try these suggestions.
Always keep at hand useful phone numbers like: police, family doctor, community care psychiatrist.
Ask well in advance to your community care psychiatrist which hospital to call for emergency admission and keep its telephone number at hand.
Know which relative or friend is closest to the patient, the one who is most influential to him/her, the one s/he listens to, the one to call even at night in case of emergency.
Know who you have to call to look after the children at home during the transfer to/from the hospital for emergency patient's admission.
Keep at hand some useful emergency drugs (ask the community psychiatrist for suggestions).
Speak to the patient calmly and firmly, and try to explain to him the emergency hospital admission procedure, to reassure him, soothe him, relieve some of his/her panic. All the situation can be far less traumatic if every act is done after a careful explanation.