For this to work, the ideal living situation is for the pair of you to be living together, either on your own or in a kink-friendly environment.
Start wearing, wetting and messing 24/7 immediately. There’s no reason for you not to. If you need to phase it in, use this rule: every time you wear a pair of underwear from now on, you have to throw it out afterward. Eventually you will only have diapers left. You must not use the toilet in any event.
Have locks installed on any room in your house which contains a toilet or a drain. Seriously. There’s the danger that if you start regretting wanting to untrain, you’ll start covertly using whatever toilet you can get. Not just the toilet, but the shower, bath, the sink in the laundry if you have one, the sink in the kitchen (that would be gross, but you might be desperate). You should only be in the back yard with your Daddy’s supervision.
Your Daddy needs to be involved in as many of your changes as possible. If you’re changing yourself, out of sight of your Daddy, that means that you can probably cheat and use the toilet, which you might do on a short-term impulse, but which neither of you actually want. Get a pair of plastic pants with a locking waist chain, so your Daddy can trust you out of his sight for at least short jaunts.
You need to be as undressed as possible around the house. It’s not acceptable for you to be shy about your diapers. You shouldn’t be in more than your diaper, a onesie and plastic pants. Ideally, just a diaper is best.
Your Daddy needs to get good at diaper checks. When you settle on a few diaper brands to use regularly, your Daddy should be checking you visually and manually (with his hand) when you’re mostly undressed around the house, to get an idea of what your diaper feels like when it’s damp, wet, soaked, messy, etc. He also needs to get an idea of the visual difference between ‘bulky but dry’/'bulky and soaked’, 'bulky but clean diapers’/'slightly messy diapers’/'very full diapers’, etc., both without and with clothes over the top.
Don’t ever ask to be changed. If you ask and you get changed, that means you can just hold it until you get changed, which you will very likely subconsciously want to do early on. Your Daddy should be checking you pretty much at random and should have a general idea of how long a diaper lasts when it’s on you. Your Daddy also should not change you until your diaper can’t take any more use; this might lead to a few rashes but that should teach you not to delay using your diaper.
Protect your surfaces. You should be travelling with an adult-size bed pad and/or puppy pad at all times. There should be one on your seat on your sofa, on your dining chair and on your office chair. There should be a PVC sheet on your bed. Ideally, there should be a removable drain filter and handheld showerhead on your shower (as well as decent water pressure). You should wear at least one extra layer of clothing to take off and tie around your waist in the event of a leaky diaper or very puffy butt. Basically, you should not feel like you need to avoid using your diaper because it would be too inconvenient and damaging to do so. Speaking of which -
You need to leak. If you take long road trips, this is an excellent opportunity. Either way, your Daddy needs to delay changing you long enough that you soak your diaper too much and it leaks from overuse. You need to know what leaking feels like and you need to get used to it, because it will be part of your life.
Do stimulant laxative training. This is something I recommend to all untrainers now. Even if you don’t want to untrain messing, using laxatives makes untraining wetting easier by making wetting seem much less important and much less painful. Stimulant laxatives are a class of laxatives which are important because they are the only type I know of which will actively force you to have an accident. There are two types I would recommend.
Dulcolax suppository training. These hit fast and hard. I would say the longest you will be able to keep control after insertion is probably two hours. Your Daddy should give you one of these just before you go into a situation where you wouldn’t be comfortable messing; for best results, you should be spending that time more than two hours away from easy access to the toilet. You need to get used to filling your diapers and you need to get used to the sensation of not being able to control it.
Castor oil training. This takes longer to hit, but after it hits, you will have no control for several hours. This will help you get used to spending extended amounts of time in messy diapers, and lets your daddy figure out the difference between slightly messy, moderately messy, and very full diapers.
Don’t be afraid to move as you need to. The body position that works for continent people using the toilet is not the same as the body position that works for soon-to-be-incontinent people using their diapers. I’m completely incontinent but I can still feel it happening (I can always feel messing accidents happening but can’t stop them, I can sometimes feel wetting).
I know that if I’m bent over, at certain angles pressure from my body can block my urethra (don’t know how to explain it because I’m sure that’s not what’s physically happening), so when I realise I’m peeing, my response is usually to lean back and open my legs. I feel it frees up any physical pressure that might be slowing down my wetting.
Position is even more important when messing. Continent people are used to sitting on the toilet, legs shut or semi-open, to poop. I promise you that as a diapered, incontinent person, the best thing you can possibly do for yourself is when you know you are about to mess, space your feet, keep your knees open, and squat deeply, exactly like a baby would do. It frees up space around your butt in your diaper (this is intentional), and makes messing smoother, quicker, less painful, less effort, and harder to stop.