Hi Arne - there is a simple form which you are supposed to be given when you get your Padron, but unfortunately they seldom give them out without being asked - at least in our local town hall at Orihuela Costa.
Take your Padron to the town hall and ask for the form. You need to sign it in front of the staff and they should then register you.
I'm not sure what else you need, but I would take your passport and residencia, with copies, just in case.
I am taking a client to do her residencia and voting form this week, so if there is more than that required I will post it on here.
It is absolutely vital that all those who are eligible register for the vote. Here in Orihuela Costa, the council are able to justify ignoring the needs of the expat population because so few of us appear on the voting register. They are only interested in winning votes and staying in power, so they need to know that we can vote against them next year if they continue to spend all the budget in Orihuela City and ignore the other half of the municipality.
Round Town News is working hard to get the message out and hopefully there will be an event in Orihuela Costa to highlight the issue, but we don't have much time to spread the word!!