Before you can write your letter, you'll first want to do a very close, analytical reading of some of Seamus Heaney's poems you read in class. I would pick two or three of your favorites and focus on those. When doing a close reading of a poem, you are analyzing it in order to both better understand its deeper meaning as well as pinpoint any criticisms you may have. You want to focus on words that you feel stand out and ask yourself why they stand out. Do the words paint images for you, or do the diction choices create a deeper meaning for you? As you look at the words, pay attention to what you know about literary devices, such as figurative language, imagery, and diction, and think about how the poet can be using these devices to relay his/her meaning. While looking at the words, you also want analyze the tone of the piece: Is it happy? sad? Romantic? Hopeless? Angry? Think about how the tone helps you see the deeper meaning in the poem. Also look at the poem's structure, such as form, rhythm, and rhyme scheme. Finally, try and establish the theme of the poem. Once you understand the theme and all of the techniques and devices the poet used to establish the theme, you are then in a better position to say what you do and do not like about the poem and why.
If we look at the poem "A Kite for Aibhin" as an example, one thing we can notice is that it is composed of 6 stanzas made of 3 lines each, except for the last stanza. The last stanza is only one line long. Ask yourself why; is Heaney trying to emphasize this last, special line, and what does it tell us about the poem's theme? Essentially, the poem is about flying a kite. It's full of beautiful imagery, such as "[p]ale blue heavenly air," "white wing," and "breeze." It's also full of diction used to capture the kite's ascent into the sky, such as "[r]ises," "[u]nspooling," and "[c]limbing." But the final line speaks of the kite taking off all alone in a "windfall." In one sense it seems the rising of the kite can represent ambition, particularly ambition fighting against obstacles, such as "briar hedges" and "stripped thorn." So is the moment the kite breaks from the flier's grip a happy moment, representing the height of ambition, or is it a sad moment representing the consequences of ambition? It's a bit ambiguous, but one clue might be seen in one of the only lines that contains an actual rhyme scheme. The second-to-last line contains an internal rhyme scheme that rhymes "breaks" with "separate" and "elate." The word elate means happiness. Who is it that is happy at this moment, the child flying the kite because it broke free, or is it the kite that is elated because it broke free? I might argue that Heaney is portraying both flier and kite as being elated by this moment of achievement and freedom and that the poem is praising ambition, but other readers may see it differently.
Hence, once you've decided what you feel the poem means, you can then write your letter to Heaney addressing whether you did or did not like the poem and what you did or did not like about it, such as theme, imagery, diction, and rhyme scheme.