For security reasons, you first need to arrange your full withdrawal via your customer center under the menu item "Orders" by creating a full withdrawal order. To do this, you must enter your entire account balance as the payout amount.
The rest is then done completely automatically. There is nothing left for you to do.
Afterwards, your securities account will be set inactive free of charge. If you ask for complete closure and termination, please write an e-mail to firstname.lastname@example.org.