William Blake's poetry spans a wide variety of ideas and themes, so it's hard to say objectively how he shows all of these (doing so would require many volumes of print). I can, however, describe generally how Blake constructs the central idea in his poetry of The Songs of Innocence and Experience. 

In these poems, Blake explores the differences between how one perceives the world in a youthful state (innocence) and how one perceives the world in a state of maturity (experience). He primarily does so by offering contrasting poems that deal with the same subject, albeit with differing perspectives. For instance, Blake wrote two poems titled "The Chimney Sweeper." Both describe chimney sweepers, but one views the occupation from a youthful, idealistic viewpoint, while the other views it from a more cynical perspective that takes into account the oppressive forces that drive children to such an occupation. By exploring the world around him with such contrasting perspectives, Blake masterfully illustrates the differences between youth and maturity.