"To a Skylark" is a poem written by Percy Bysshe Shelley in June 1820. It consists of twenty-one five-line stanzas. The stanzas each consist of four lines of iambic trimeter followed by a line of iambic hexameter. The rhythm generally follows the metrical scheme, with the more common variations including acephalous lines and spondaic substitutions. The rhyme scheme is ABABB. The majority of the lines are end-stopped with prominent rhyme words—some monosyllabic with long vowels but a few with feminine rhymes. The relatively regular rhythm, the use of trimeter, and the end-stopped lines give the intended effect of a song, mimicking the song of the skylark.

In the poem, a first-person narrator uses the rhetorical technique of apostrophe, personifying the bird and talking to it as though it were a person. As is typical of Romantic poetry, the narrator sees the bird as an emblem of pure joy that is "better than all treasures / That in books are found."

Before tackling this question, it helps to understand what a critical appreciation is. A critical appreciation is a knowledgeable evaluation of a piece of literature or poetry. For instance, the reader must have enough relevant knowledge about the particular piece in order to construct an informed opinion about it. It's worth noting that a critical appreciation can like or dislike the piece of literature being examined; the important thing is to know enough about the work to give good reasons for why you like or dislike it. 

With this knowledge in mind, let's look at "To a Skylark" by Percy Bysshe Shelley. Your critical appreciation will depend on the particular context from which you approach the poem, and also on what you like or dislike about it. It's still possible to provide some examples, though. For instance, you could say you like the poem because it skillfully uses a skylark as a metaphorical representation of the natural world. Conversely, you could say you dislike the poem because its archaic language makes it difficult to read and comprehend. Whatever the case, you'll need to back up your particular opinion by offering a careful and insightful analysis of the poetic characteristic you're discussing. Your opinion does not necessarily matter, as long as you back it up with a sound evaluation of the work in question.