Greeting: it is the way to say hello to somebody. Remember it's a formal letter! Don't say hello, what's up and all that stuff!!
Opening: explain why you are writing the letter briefly (where you bought the item, date...)
Issues: why you are upset, details about what went wrong with the item you bought
Complaint: That's the key part of your writing. Express your disatisfaction . Don't use vulgar words, threats or swear words!!
Demands & Suggestions: They are very similar , but not fully the same. Demands is about stating what you are expecting them to do, how they will make up for your disatisfaction (how they can compensate you). Suggestions is about posing possible solutions to your problems.