If you use a cash register, giving the correct change will be very simple. You will simply enter the cost of the product, the amount that the client gave you and that's it. The cash register will tell you how much change you have to give. However, if your cash register is damaged, if you entered the wrong amount, or if you don't have one, you will need to know how to make a change yourself. The basic method is to count from the purchase price to the amount the customer gave you.
Steps
Method 1 of 2: Give back
Step 1. Make sure the change you give and the price of the product together add up to the amount the customer gave you
You must make the customer leave with the same value that they gave you, only now part of that value will be the product and the rest, the change. It's that easy. For instance:
If you were given $ 20.00 to buy a $ 5.00 book, then you will give a $ 5.00 value book and a $ 15.00 value change, for a total of $ 20.00
Step 2. Add the amount that the client gives you
Before you can give the change, you will have to know how much money he has given you. As you count the money, put it in the cash register or on the table in front of the two of you. When you finish counting, indicate the amount he gave you. This will ensure that there is no confusion or disagreement regarding how much money the customer gave you.
Step 3. Count from the amount of the product purchased to the amount paid
For example, if the customer bought a sandwich that costs $ 7.59 and gave you $ 20.00, you would start at $ 7.59 in order to give him his change and so you would count until you reach $ 20.00.
Step 4. Count out loud to avoid confusion
It is not necessary to list every coin, but it is important that you at least say the total each time you reach the end of a particular unit (in the United States this could be done with the 1, 5, 10 or 25 cent coins). With banknotes, mistakes are more costly; therefore, a good idea is to keep a running count.
 For example, if you were given $ 10.00 for a $ 6.00 product, you would do the following:
 Count the $ 1,00 bills and then give the total, like this: "One, two, three, four and you have ten."
 Or add as if you were continuing the count, like this: "Seven, eight, nine and ten."
Step 5. Start with the coins
Start with the lowest value coins, for example, in the United States you would start with the 1 cent coins and then with the 5, 10 and 25 and then continue with the bills. If you flip backwards (that is, starting with bills and ending with coins), it will be rare and your customer will likely leave the change since they will already be holding the bills. If your customers leave the coins when you give them to them, this is likely the reason.

In our initial example you start with $ 7.59 (the price of the sandwich); therefore, you would give back:
 a 1 cent coin (“you have $ 7, 60”)
 a nickel (“7, 65”)
 a dime (“7,75”)
 a quarter ("8.00")
 While this is the most efficient combination of coins in the case of the United States, it doesn't really matter how you add up to $ 8.00 as long as you get to that amount.
Step 6. Then go on to give the change in bills
Once you've reached a full amount, start counting the bills until you reach the amount the customer gave you. Back in our example, it would look like this:

You've already reached $ 8.00 and you have to go up to $ 20.00; therefore, now you would give back:
 two $ 1.00 bills (“9, 10”)
 a bill of $ 10.00 ("and 10 more and you have 20")
Step 7. Check your work
You gave the customer $ 0.01 + $ 0.05 + $ 0.10 + $ 0.25 = $ 0.41 in coins as change. Then you gave him $ 1.00 + $ 1.00 + $ 10.00 = $ 12.00 in bills, for a total of $ 12.41 in return. $ 7,59 + $ 12,41 = $ 20,00 (this is the amount the customer gave you).
Method 2 of 2: Handle More Complex Amounts Paid
Step 1. Be prepared for customers who give you strange amounts in order to receive less coins or certain denominations
For example, if the total is $ 6.00, a customer might give you $ 11.00 in order to receive a single $ 5.00 bill. Otherwise, if that person gives you $ 10.00, they would get four $ 1 bills back., 00.
Step 2. Add as you did before in order to have simpler transactions
The sum will generally be simple, especially with transactions that do not involve coins.

For example, if the customer bought a hat for $ 42.00 and gave you $ 47.00, you would add like this:
$ 5.00 bill: "You gave me $ 42.00, plus $ 5.00, so you have $ 47.00."
Step 3. Consider doing some subtraction first to make things simpler by having more complex transactions
How to get from $ 12.78 to $ 23.03 may not be immediately obvious. In this case a little initial subtraction can simplify things:
 Start with the amount the customer gave you. Subtract from that amount to get a simpler number. In this case, it would look like this: $ 23.03  $ 0.03 = $ 23.00.
 Now subtract the same amount from the price, like this: $ 12.78  $ 0.03 = $ 12.75.

Now it will be clear that you will first have to give a quarter and the change would be like this:
 a quarter (“you have $ 13.03”). You will go from $ 12, 78 to $ 13, 03.
 a $ 10 bill ("plus $ 10.00 and you have $ 23.03")
Step 4. Confidently flip the correct change for any combination
As another example of a more complex situation, imagine you are a waiter and a customer buys $ 112.31 worth of food. The customer will give you six $ 20 bills, a nickel, and a 1.
 Add up the amount the customer gave you when counting the money: 20, 40, 60, 80, 100, 120, and 6 cents. Tell the customer the amount they give you: "$ 120, 06".
 The client gave you a rare amount of money; therefore, this case could be a good time to do some subtraction. $ 120.06  $ 0.06 = $ 120.00 and $ 112.31  $ 0.06 = $ 112.25. Better. Thus, you would need three quarters.

Now start adding from $ 112.31 to $ 120.06:
 three 25cent coins (“you have 113, 06”). We know from our previous subtraction exercise that this method does work.
 two $ 1 bills (“114, 115”)
 a $ 5 bill ("and 5 and you have 120, 06")
 Check your work. You gave him $ 0.25 + $ 0.25 + $ 0.25 + $ 1.00 + $ 1.00 + $ 5.00 = $ 7.75. $ 7.75 + $ 112.31 = $ 120.06 (this is the amount the customer gives you it gave).