I say "yes", because I don't want to be rude to people and mostly when I say "no", I feel that person feels bad about it. There might be different cases for saying "yes", but in my case, I was thinking that saying yes to an offer, question or a request makes the thing easier.
But saying "yes", can sometimes cause problems, like burn-out or bad feelings. In some cases you might end up with saying "yes" to a case which you can't handle.
So being able to say "no" to certain things can help to maintain your mental health. As well as provide better decision making in long-run.
Note that all of these examples & practices are to develop your "no" muscle. You should go as wild as you can with the simulations.
I imagine myself going down the street in a chill mode. Some old lady requests me to help with carrying her bags.
I imagine saying "no I am in a hurry", doesn't matter if I am or I am not. It will train my "no" muscle. I imagine the her saying okay and requesting help from the next person.
I imagine being surfing on the internet and my friend texts me that I need to fix/do something. I imagine saying "no, I am exhausted today, I'm in my chill time, I'll look at it tomorrow"
I imagine my sister wanting me to get up and check the food in 10 minutes. I imagine telling her "no, I am concentrated right now. You need to figure out another method which doesn't involve me"