Example:
6/20 - 1/5
First, you need to find out what the least common denominator (lcd) is.
The lcd is 20 because 20 x 1=20 and 4 x 5 = 20
So ...
6/20 = 6/20 and 1/5 = 4/20
then you take the new fractions and subtract them
6/20 - 4/20 = 2/20
Lastly, if possible you simplify
2/20 =1/10
Hope this helps! I tried to make it as least confusing as possible.