Complaints can be oral or written.
Oral complaints can be made in person or on the telephone.
It's always best to address the problem as soon as possible. Fake fingernail in your fish-n.chips? Talk to the restaurant manager. Got home to find out the milk you just bought is spoiled? Call the supermarket and tell them (you'll probably have to bring it back for a refund or exchange). Amazon sent you the wrong color prom dress, and it's too late to send it back? Call them and lodge your complaint for their incompetence!
Written complaints can be make via letter, online form, or email.
Written complaints are good when 1) you have already exhausted all other avenues (you've talked to the manager, you've called the head office, no one is listening to your silly complaint) or 2) you need a paper trail. This is particularly true in business. If there's a problem with a shipment (lost, late, damaged) you need to put your complaint in writing so people know what the problem is and what action has been taken.
As with all letters and above all, business letters, POLITENESS is key. Don't say "You ruined my holiday!" say "I'm very disappointed that the room I booked for my honeymoon was decorated with pictures of military uniforms and swords crossed over the bed."
Salutation (Dear Sir or Madam is fine, but if you can find the person responsible for complaints, or an address for the company president, that's better!)
Introduction: What is your connection with this company? "I booked a week at your spa from January 8 to 15..." "I recently ordered a fishing pole from your online store..."
(TIP! You might want to start off with a compliment)
Reason you are writing: What is the problem? Be specific (but don't lie). Don't just say the food was cruddy. Say that the chicken vindaloo was so spicy that you actually had to see a doctor for the burn on your tongue. Don't say that the room hadn't been cleaned. Say that the toothbrush and toothpaste from the former guest were actually still on the sink when you entered your hotel room. Make sure you explain how you are personally affected by the problem.
Suggestion of what could be proper handling of the situation: You can suggest a replacement, your money back, a voucher for your next purchase, that the company does something to improve the situation. Just make sure that your suggestion is in line with the actual "damages."
Polite close: You hope that the situation can be settled to the satisfaction of both parties. Don't threaten with legal action unless it is something that is really meaningful "Seeing that we paid €1200 for security at the ball, and at least half of them were drunk, we seek reimbursement of €600. If a compromise is not forthcoming, we will be forced to take legal action."