To apply into court as a Mac you first should ring the court concerned and find out whether a formal application is needed or not.
My experience is that some courts allow Macs without any formal need for an application and others do not. You just have to follow the proceedure set by the court.
I believe in family proceedings the form you need to complete if an application is needed is a D11 but check with the court.
What needs to go with the application is a short CV giving brief details of the person concerned and also that the person does not have any financial or personal relationship with the defendent that would be prejudential to proceedings.
I think that you can find details of the guide to being a MacKenzie Friend somewhere on the net (I found it once as I go to court with a MacKenzie friend who is a neighbour of mine not a professional Mac)