Hey there, well I did Yeats for last year's HSC and basically, just know the main themes of the poems, know two in detail because you'll most likely be asked to make some sort of comparison, and know a bit about his background as well because that will help make sense of the poems.
You don't need to know every single poem, but know a few pretty well and memorize some quotes that represent the main themes in the poems. Then you'll just have to apply them to whatever question they give you.
You might not be able to do both Byzantium and Sailing to Byzantium, so study them both but study a third one as well in case they ask for a comparison between a different poem and one of the Byzantium ones.
Hope that helps, can't remember much cos it was a year ago now, but good luck