Consistent corrective actions, my boy used to love getting on his little table, which he’s not allowed and very well knows he’s not (that cheeky smile and side look tells everything haha), so every time it’s a strict no and getting him off inmediately. If he gets back on again, I flip the table. Used to be drama initially but then he just understood that that’s how it’s going to be every time and eventually stopped completely. The corrective action can be removing from situation, taking away a toy for a moment or similar. Can take them a long time, but eventually they’ll know what happens after a ‘no’
Worth reading Toddler Calm by Sarah Ockwell-Smith. Explains the toddler developmental ages really well and how best to help them understand what you need them to. I found it very helpful and insightful!
I was given a small book from the children centre and it's called gobrain toddler book, it's free! It's so simple and easy to understand. The guide teaches you about their age, brain and that at this stage they really don't understand much, but they are growing and learning and no toddler is naughty but just they are exploring, act on impulsivity, and their brain isn't the mode of fully understanding and they have no clue how to self regulate. Try also YouTube from professional toddler experts about behaviour, I've also learned a lot from this. I think it's about being calm and understanding how your toddlers brain works as they grow..hope this helps.
It's not age-appropriate to "discipline" a 20-month old child. It's much too young for time out as they're not cognitively able to understand it. Try your local library or bookshop for some books on toddler behaviour. It's an awful lot easier when you know what's going on for them and what's developmentally normal! 😊