I think I replied to your question about Form FL 423 on another forum
earlier today.
You need to state any information or belief that
supports your application that you are the parent and quote the sources of
your information or belief. Then declare a statement of truth - "I believe
that the facts stated are true."
I don't think it needs to be
signed in the presence of a person who is authorised to administer oaths
but you can check that with court officials because living in Scotland
where procedures are different I'm not sure now.
If the
officials don't know ask them to check the Family Proceedings Courts
(Family Law Act 1986) Rules 2001