I agree with Ričardas' answer, but I also think listening to lots of English audio (music, documentaries, podcasts) is very useful, because if you listen to the same audio over and over, you will begin to notice patterns in the language, which should begin to help you with prepositions too. :)

When learning a language, there is no such thing as "the easiest way" to learn, you have to dedicate your time, determination for learning a language. The best way to learn prepositions is just to search for a list of prepositions in google, memorise them and then do some exercises with prepositions which can be also found in google, try it.