Hi! Former KFC cashier here!
For full-time, the first week (five days) is going to be the training. (For part-time, that would take ten days, a.k.a. two weeks.) While in training, you will not be allowed to take orders by yourself. You will always have someone near you. If you are lucky, you will have a trainer. If not, you will be trained by older cashiers. Until your training is over, your duty will literally be to assist a chosen cashier or multiple cashiers with their orders until you learn the ropes.
In the end, you will take a final test. For it, you need to know the menus, what they contain, the buckets and what they contain (that falls under menus too!), what the salads contain (including how many grammes of something they have), and what offers/promotions KFC has during that time.
You will also do some online training, which is reaaallly simple.
An important note: learn how to make the ice cream! It is trickier than it looks. It took me over three months to get a decent-shaped ice cream, although most others get them right as soon as they make them.
After your training is over, you will be able to take orders by yourself. You will have your own register and keep in mind you need to be as accurate with the change you give as possible because they might make you pay if you are, say, 10 pounds short one day. Chances are they will not do that, but try to be accurate anyway so as to cause no trouble.
Sometimes, a customer will get a special ticket that allows them to give feedback to the restaurant. When this customer comes in, MAKE THEIR ORDER AS QUICKLY AS POSSIBLE AND MAKE SURE IT IS PERFECT! Do not forget to smile and be the best cashier ever with these customers because if they give your restaurant a rating of one star, your manager will question you about it.
There are (many) times when the cooks will not hear you when you ask for a sandwich (let us pretend a customer orders a Tower Zinger in this case). You have to tell the cooks 'Tower Zinger' so they know to make it. However, they may not hear you, so be prepared to shout it three times and wait up to five minutes for it occasionally.
Dipping fries and all sandwiches besides the Booster take three minutes to cook. If someone asks for dipping fries or a sandwich other than the Booster and you have none ready, you must tell them they must wait three minutes for them before they order.
If you know that a flood of people will come in at a certain hour, it is a good idea to pour some drinks (mostly Coke) before they come in order to reduce the time it takes to complete orders.
There are *certain* customers who are less than pleasant to deal with. One started a rant about how our fries are the worst after I recommended him the menu for a whatever he ordered (we HAVE to recommend the menu!). You just sit there, smile, and do not argue with them if this happens.
Also, a few people do not know the difference between KFC and McDonald's. Someone once asked for a McChicken and another person asked for McNuggets, to exemplify.
I strongly advise that during the school year, you work part-time rather than full-time, and that you quit before your exams. My sister went to school full-time and to work full-time in year 12 and she ended up with more or less average grades. She was always tired or busy and could hardly find time to sleep. In the end, she switched to part-time because she could not handle it anymore. I myself quit my job before my exams because there is no way in hell I would have been able to get any schoolwork done while working. I was always tired and never had the will to do anything, which affected my revision greatly (to be read as 'I did not get to revise at all'
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During the holidays, of course you can work full-time if you have no school, but keep in mind it will leave you exhausted. My friend, a KFC cashier who worked full-time during the holidays, described it as hell since it is going to be so busy.
Good luck at your job and with your studies! If you have any other questions, feel free to ask