With thanks to other posters on various forums. Corrections welcome.

The paperwork
Basically you complete the FN by telling them what steps you are taking ie agree to end any dispute or appeal you have with them.
You then tell them how much you are going to pay.
You then sign and date the FN and submit it before the corrective action date.

The money
You can then ask for any CTD to be allocated to your CTD to the APN references
You can send a cheque or make a payment via BACS \ Faster Payments etc.
You can ask for TTP arrangements. HMRC are discussing the possibility of 5 year TTPs. They should also research and potentially invoke "hardship". The criteria for meeting hardship is lower than you might think.
UNDER NO CIRCUMSTANCES LET ANYONE IN YOUR HOUSE THAT IS NOT THERE WITH A COURT ORDER.
If you're facing bankruptcy, TAKE ADVICE FROM AN INSOLVENCY PRACTITIONER.

Afterwards
If everything is in order, they will then send you details of the amount of interest and surcharges \ penalties you must pay and the date they should receive this. Once you have done this, you should get confirmation that your tax dispute has been settled in full. If you dont get this, you need to ask them to send you confirmation of this so they do not come back at a later date and ask for money \ send you penalties and surcharges.