I've had many people (customers) try to tell me how to do my
job. Generally, I'm polite to them, but there have been cases that I've been
surly.
One example - When I worked at Subway, I had someone telling me exactly how to slice the bread, then exactly how to fold the meat and cheese on to the bread. Then they said, "Put only one squirt of mayo, no, don't squeeze that hard on the tube, oh, that's going to be too much!". I wiped some off. If the bread, and the meat, and the cheese, and the mayo wasn't enough, they also got upset about the tomatoes.
"Five tomatoes! You must put on five tomatoes! You only gave me four!"
Then, this guy started in on the amount of pickles. There were two other people waiting, and they were getting pissed at this guy for taking so long. I lost it. It was late, I was cranky, and I wasn't going to take any more.
So, I said "Look buddy, if you wanted this sandwich made a specific way, why didn't you go to the store, buy bread and meat, and make it yourself?". He just stared at me dumbly. That dumb stare made it worse. I got even more testy and said, "If you want, I'll give you my apron and you can come back here and make it yourself since I obviously can't get it right. You can make the sandwiches for the other people in line too if you want."
He got all pissed off and yelled something incoherent, then walked out. I deposited his sandwich in the trash can, and asked the next customer what they wanted.
While this is a funny story, if you want to keep your job, you should not follow my example. In this case, I already had another job lined up and I wasn't too worried about being fired since I had given my notice a week before. Generally though, being surly to a customer, even if they deserve it, isn't a good career move. Most customers will complain to your boss, and you'll end up reprimanded or fired.
It sucks to be told how to do your job, especially by someone who wants you to help them. The only thing you can do most of the time is shut up and deal with it. If you can't handle customers, you shouldn't be in customer service.