These are my suggestions based on the last three months.
Identify what the problems are
(1) Physical control - how long can the child stay dry/clean in the day?
(2) Lack of understanding of what is expected on toilet or potty - this is often the key problem with SN children whose communication/understanding is not age appropriate.
Problem 1. You would probably want a dry period of 2-3 hours in the day. I didn't have any problems here as DS1 was dry for long periods of the day so I don't know what is advised other than waiting if the child still has control problems.
Working with Problem 2
In the first few weeks take the child frequently to the toilet. Make it a pleasant experience and reward him/her with a treat. However don't prolong the visit if there is no result as otherwise the child might start to think 'this is just a place I go to get treats'
Try to make a rough note of the times of the day that the child is wet or dirty. This will never be an exact science but might give you some idea about times of the day to focus on.
When you think the child is familiar with the procedure of going to the toilet ( and has positive associations) you can reduce the number of visits in the day and try to work on a pattern of first thing in the morning, before leaving home, when arriving back home, 20 mins after meal, before bed time etc
General issues
Go cold turkey with removing absorbent nappies. Until the child is aware of the problem of wetness, there is little incentive to solve it. DS1 was wetting the kitchen floor for a few weeks before finally realising what the toilet is really for.
Patience, Peristence Perseverance - The process will take a long time compared to what is involved with an NT child. Don't tell anyone you don't trust that you are doing it. They may be well meaning but they might not realise just how much harder it is when the child is developmentally delayed. The last thing you want is little anecdotes about what they did with their NT children. Also the time scale for NT children is much shorter so any comparison will make you feel doom laden.
It seemed to click more with DS1 both when he realised he did not like being wet and also when we let him stand on a step by the toilet so that he can face forward like daddy. He is training daddy now - when he takes him into town he says 'Daddy toilet' and starts taking him in.
Well we are only 2 days in and I am sure there will be setbacks but I feel a bit more confident now about eventual success.