Once you have chosen the ballot paper and put it into the ballot envelope (or not put it in if you want to cast a blank ballot), you have to seal it and then you have to put the envelope containing your vote together with the certificate of registration in the electoral roll in the envelope addressed to the chair of the committee where you have to cast your ballot.
The envelope addressed to the chair of the committee is the envelope you have to send by registered post no later than 23 September. (Extension of the deadline: until 24 September. Exceptionally, up to 2 pm on 25 September in those places where 24 September is a holiday)
You do not have to pay any postage.