You must declare the withdrawal in text form. Text form means a letter, email or fax, each with a clear identification of the contract and a clear declaration of withdrawal. A verbal statement is not enough.
We recommend registered mail with a return receipt or an email with a read receipt. Both create proof of delivery, which is decisive in disputes about deadlines. Under § 8 (1) VVG, sending it in time is enough to meet the 14-day period, but you should be able to prove that it reached the insurer.
A short sentence is enough, for example: I hereby withdraw from my insurance contract no. [contract number] dated [date]. A detailed explanation is not necessary.