The following image was made by Subtract Fractions With Circle Models Designer:

The parts of a subtraction example are the minuend, the subtrahend, and the difference.

When the program starts, you will be asked to identify the minuend in the first row and then the subtrahend in the second row. The program will continue when the minuend and subtrahend are correctly identified. You will then be asked to find the difference.

You can see from the picture that the minuend has 3 1/9 units and the subtrahend has 2 7/9 units. The difference is what is left after removing the blue circles in the second row from the circles in the first row. In many examples, you can arrive at the difference by looking at the picture. For example, after removing the two whole circles, you are left with 10/9 red circles. Removing the 7/9 circle from the 10/9 circle will leave 3/9 circle.

When the numerator of the subtrahend is larger than the numerator of the minuend you may rewrite the minuend to subtract. Some texts call the procedure "borrowing". To borrow, simply decrease the whole number 3 by 1. Then add 9/9 to 1/9 to get 10/9, giving the fraction 2 10/9.

Notice in the above image that 3 1/9 is renamed as 2 10/9 so that the numerators can be subtracted.

You may prefer to work vertically:
Another method would be to write the minuend and subtrahend in fraction form and then subtract the numerators. The example would look like this: