The first step is to think, what is the first number that both 5 and 8 go into. We want to get them both to that number so we can have a common denominator to work with. In this case it's 40.
We need to multiply the side with 5 by 8 to get 40, and the side with 8 by 5 to get 40.
A shortcut to do this is simply to do what they call cross-multiplication. We multiply the bottom of the first fraction by the top of the second, and the top of the first fraction by the bottom of the second.
Now you try the rest of them.