Hi OP. Is your ex legally represented? As he is the applicant, it would be his responsibility to prepare the court bundle. In addition, where one party is represented, that party should prepare the bundle even if they are the respondent. However, if he is in person too, your main goal is to try to get something coherent to the judge.
Normally you would only need to lodge a bundle if the hearing is listed for more than an hour. Normally the bundle would be in an A4 lever arch file with the sections separated by cardboard/paper dividers. Each divider would have a number- A,B,C,D,E etc.
The sections would normally be split as follows for your type of case
A- position statements, chronology, case summary (ie documents prepared for that particular hearing that give the judge a snapshot of the case)
B- applications and orders- ie copy of the C100 and any supplemental forms, copies of any orders made by the court, in date order
C- statements and affidavits- ie witness statements from you and ex
D- reports (ie Cafcass report, any medical reports ordered by the court)
E- any other documents although you should only include things that are genuinely relevant. Correspondence is normally not relevant (if it was you would have exhibited it to your witness statement)
Number all the pages throughout (don't start a new number for each section). Here is a link to the practice direction on how to present the bundle. www.justice.gov.uk/courts/procedure-rules/family/practice_directions/pd_part_27a#para4.1.
Make one copy for you and one for the judge. Ideally, your ex should agree the contents of the bundle before the hearing.
I really really recommend the book suggested upthread by Lucy Reed. Do get hold of it.
I would caution against using a McKenzie friend if they charge money. I think it is unethical to charge people money when you are not qualified and often do not know what you are doing. I have acted as McKenzie friend before (for free) for a friend. I am legally qualified and specialise in this area but there are many McKenzie friends out there who have little or no legal training.
It is easy to say 'just get a solicitor'. Ideally you should, yes. But solicitors would charge around £5,000 plus VAT for a final children act hearing. How do you magic that up? Especially since it could be better spent on your children. This is the sad. sad reality of cutting legal aid funding. Five years ago, you would have gotten legal aid no problem. :(