You can start offering toilet opportunities whenever YOU feel ready and there aren't any big distractions around. It's best to start when you are both feeling healthy and relaxed, including by setting the space up to be quiet and calming.
Consider what they are capable of already by themselves, and how much you need to assist and what preferences baby might have (options you provide for where and when to do it).
When to offer
You choose when to offer a toileting opportunity based on a combination of:
Your toddler's signals that they need to go (body language, noise/grunt, a pause, a facial expression, fussiness, wriggling, squirming, pre-poo farting)
Your toddler's timing (intervals between feeds, pees, etc you've observed)
Your intuition (your thoughts of "its been a while, lets try")
Common transition times (when most toddlers typically need to go), e.g:
After waking up from a sleep/nap - just like us adults!
After eating - the body making room during a meal, or soon after
After taking their nappy off (nappy change) - when they feel bare bottom freedom and have been holding on as to not soil themselves
After taking the toddler out of a stroller, carseat, highchair, babycarrier - if they still have their instinct not to soil themselves then offering when transitioning can be a good time when you've also got their attention. It is also useful to offer before going into one of these, especially if for a longer time, for obvious reason.
Choose what would work for you, start small - offer a few times a day and then gradually build on that.
Another easy and very rewarding opportunity is to offer whenever you see a clear poo signal ('that' expression). Most parents know when their toddler is pooing. Ask them to "wait", then move them calmly onto the potty and help them relax to finish there. This sets good habits early on that poop goes in a proper place. Even if you only ever catch poo, that’s success.
Don't feel discouraged if you aren't seeing any signals (many caregivers don't, especially for pee) as you can still offer toilet opportunities based on timing and intuition very successfully.
How to offer
1. ANNOUNCE - Use a simple hand-signal or phrase (e.g. 'it's toilet time') to tell your toddler that you are about to offer.
2. POSITION - Get them into position - use a hold that suits you both.
3. BREATHE - Lead them by taking a deep breath in and out to relax, then make your cue sound (e.g. 'pssss') for 5-10 seconds to let your toddler know that now is a good time to let go. Wait and see (up to say half a minute). If they squirm or are getting upset, stop and offer again another time.
4. EXPLAIN KINDLY- Explain to your toddler what's going and make it fun for them by using loving words of encouragement or making funny faces and smiling. If you both enjoy the process each time, whether they go or not, it builds trust and increases the chances you'll offer again. You could say "look you've peed, you've got the hang of this" or "looks like you don't have to go just now, let's try again later". Then, announce the "finish" of the offer before you move your toddler off to wind up the routine.
5. Give a quick and gentle clean up with a water and a cloth/wet-wipe as needed, then clean the potty/sink, or flush the toilet, and set up ready for next time.
You will likely be very delighted for your toddler when they pee or poo on cue – which is great as your warmth and enthusiasm makes the process fun – but it is best to stay in the realm of positive reinforcement rather than praise. None of this is to please you. The urge to pee or poo is not something someone can control, so it makes no sense to praise or reward it. It's a normal bodily function and the focus should stay on your toddler. Consider how you react to other daily activities - just as we don’t usually reward toddlers for eating or sleeping, going to the toilet is a matter-of-fact thing they'll do all their life.
Understanding what you want from each other is critical for working together - or in this case, learning together.
Every toilet learning experience should be a positive one - toddlers learn best when feeling safe and having fun!
Clothing tips for toddlers
For naps you could lay them on a flat prefold and waterproof mat and just dress their top half (blanket to cover their lower half) so wake-up toileting is fast and easy (no pants or nappies getting in the way). If you prefer a nappy on then a singles, shirt, dress, sleep gown or legwarmers give quicker access than messing with pants or onesie snaps/zips. You can also simple tuck a prefold or muslin cloth into a nappy belt as a back up instead of a nappy with velcro or buttons.