This is a somewhat difficult assignment as liberal humanism isn’t a literary theory per se, poststructuralism and postmodernism are terms used interchangeably, and deconstruction is a type of postmodernist literary theory. Your best choices are probably structuralism and deconstruction.
A common form of structuralist analysis of poetry is based on the linguistic theories of Ferdinand de Saussure, which emphasize the contrast between langue (linguistic structure) and parole (specific instances of language). You can analyse a poem by look at metrical as langue and the specific rhythm of the poem, and its relation to the putative meter as parole.
Deconstructive criticism is a good choice because it is very formulaic. Look for an obvious binary opposition in the theme of the poem – poor vs. rich, nature vs. civilization, light vs. dark, good vs. evil – and show that (1) the tow are not really opposed by interdependent and/or (2) the writer is being subversive by taking the one of the pair normally considered bad and showing how it is really good (e.g. arguing that Satan is good in Milton, that Hardy’s `Ruined Maid` is an ideal, etc.