This is such a vast subject and I am convinced that there are many others out there who have written wiser things than I will, but I had a request from my niece who is a sweet Mama to 2 small boys.
I've decided to do it in small installments and I am going to start with the wee years first.
When children are infants they need all the love and attention they can get. They like to be talked to, sung to and held. Easy stuff.
As they start to grow we begin to see their wills develop. They get mobile, and start to explore their world. They can now get into things that we don't want them to get into. We get frustrated at having to re-pot that fern over and over again. We begin to wonder how we will ever get anything done when the little darling is awake, and needing to be watched. We can't possibly spend every moment by their side, can we? I know I didn't, but I know someone who did with their firstborn. Never left the child's side. I wasn't interested in doing this. What to do?
First of all, with small children, we, of course, cannot leave them unattended for long lengths of time. But we can expect our children to learn what they can touch and what they cannot touch.
It just takes a decision on your part that you are going to be diligent to say no every.single.time. That's the most important thing.
When our oldest son was at the crawling and pulling up stage, I had a lovely potted plant on our coffee table. The dirt in it was endlessly fascinating to him. He would crawl over, pull himself up, and put his hand into the dirt and start to make a mess. We didn't want to have to clean this up constantly and we did not want to move the plant. So, every time he touched the dirt, we'd flick the back of his hand. Not really hard but enough to get his attention and we would say "no touch!" We did this every time. It didn't take long before he never touched that plant. People would come over and be amazed. We would go to other people's homes and they would say "Oh, do you want me to move that, so he won't be tempted to touch it?" And we would say "No, he won't touch it," and he wouldn't.
A key thing in all of it is to be diligent, and not lazy. I didn't always want to get up off the couch when pregnant with Emily to enforce the 'no pulling books off the bookcase' rule with Lindsay. She made a mess daily at 18-20 months that I would have to clean up. Until, I remembered that I must be diligent to train her. No more books on the floor!
The other key thing, and this is very important. Do not tell your child to do or not to do something you do not intend to enforce. They will learn quickly that you do not mean what you say, and that they don't really have to obey you. Then when you do come to your senses and start to be diligent it will take longer. But it can be done and you will be happier and so will they.
It has been quite a while since we had one of our own children in this stage. Kyle is seven now, but we have our granddaughter living with us, and we watch her when her Momma is at work. She is a toddler at 2 but the basic principles are the same. You give them instruction, expect them to obey, give a consequence for not listening or praise for when they do listen.
We may say "Kamryn, you may not throw stuff at Uncle Kyle." If she does it again, we look her in the eye, and repeat our instruction. If she starts to fuss, I tell her "Kamryn, Grandma loves you! You don't need to fuss, you just need to obey. Don't throw things at Uncle Kyle." If she does do it again, I usually just hold her and don't let her play for a while. If she tries to get down I tell her "No. You threw that book at Uncle Kyle. You can get down in a little while." When I can tell she has yielded her will and is sitting calmly on my lap (I don't play with her or read books during this time) I will quickly praise her for sitting so well and remind her about not throwing, and put her down to go play. That is usually all it takes.
Consistency, and love. Diligence and not laziness. That's what it takes.
Proverbs 29:17 Correct your son, and he will give you rest; Yes, he will give delight to your soul.