Any way you like! There is no right or wrong way.
For example you might click "I like it!" if you just know intuitively that it's good. Or maybe the loop has potential: some interesting sounds, rhythm or melody you haven't heard before perhaps.
You might click "I don't like it!" if the loop is boring, has an annoying noise or just sounds bad or wrong.
It's best not to think about it too much!
If you are new to a channel, it might help to listen for a minute before making any ratings.