I also recommend the triple p program. Also something to be aware
Of
Is that initially when you start a new behavioral technique the children wil up the intensity. Therefore tantrums may increase to begin with. Keep persevering at this point. If you give in they will be ready to go for longer in the next round. Once they see you are not afraid of a tantrum and will still follow through they will respond quicker an with less fuss. Something to remember is if you tell make sure they do, if you ask they can say no. Having in mind a consequence for behaviour before you at in the situation can also help, so I definately agree with Feioja Mum that know there currency is
Important. As for us as adults each of our behaviours have a consequence be it positive or negative. If you an the kids know consistently what will happen if they do xyz then there isn't any need to nag or yell. Easier said than done but it does get easier and more second nature the more you do it.
Hope that made sense because I'm on my phone. I think my key point is consistency is the key and makes following through easier. All the best.