Complaining well

Clear, dated, specific.

The letter

State what you bought, when, what is wrong, what you have already done, the remedy you want, and a reasonable deadline. Keep it factual and short.

Reference the legal right you are relying on.
Keep every reply; escalation depends on the record.

Escalating

Follow the firm's complaint process to its end, then take it to an ombudsman or regulator. Public posts sometimes work, but they do not create a right to a remedy.