Stevenson's poem begins by pointing. "See the smoke trail!" the speaker says, and we are pulled into a valley where bonfires burn in garden after garden. Nothing happens in the poem except that someone watches smoke rise and then makes a small decision about what it means. The decision is the point: summer has ended, but the year hasn't gone dull, because "Something bright in all!"
The first stanza sets the scene with a sense of community. The speaker's own garden is only implied, since the fires are in "the other gardens" and "all up the vale." The whole neighbourhood seems to be doing the same thing at once, and the line "all up the vale" lets the eye climb the hillside from one plume of smoke to the next. The verb "trail" is gentle and horizontal, like smoke drifting low and thin.
In the second stanza the smoke changes. The fire "blazes" and the smoke "towers," so the image moves from sideways to upward and from faint to monumental. Colour does similar work. The fire is "red" and the smoke "grey," and the plain pairing makes a bold picture, with flame at the bottom and a grey column above it. Even the grey is treated as something to admire, since "towers" suggests height and strength rather than gloom.
The same stanza handles loss with surprising speed. "Pleasant summer over / And all the summer flowers" has no verb and no lingering. Summer is mentioned kindly ("pleasant") and then set aside in half a stanza, before the fire takes over the next two lines. The sound reinforces this, because "flowers" in line two rhymes with "towers" in line four. What the garden loses is answered, in rhyme, by what rises from it.
The rhyme scheme is simple. In each stanza only the second and fourth lines rhyme (vale/trail, flowers/towers, all/fall), and the first and third lines are left open. The lines are short, usually two main beats, and they move in a skipping, tripping rhythm: "In the OTHer GARdens." That lightness suits the mood. It sounds like something a child might chant while watching the smoke, and it never slows into the heavy, sorrowful pace that poems about autumn often take.
The last stanza steps back and states the lesson. "Sing a song of seasons!" echoes the opening of the nursery rhyme "Sing a Song of Sixpence," which tells us the poem wants to be a singing game as much as a reflection. Then comes the exclamation "Something bright in all!", a short, confident statement that works almost like a proverb. The final couplet-like pair, "Flowers in the summer, / Fires in the fall!", balances the seasons exactly. The structure is the same on both lines, and the words even sound alike, since "flowers" and "fires" share their opening sound and nearly their shape. Sound makes the two seasons seem like equals, and the fire reads as a transformation of the flower rather than a poor replacement. The word "fall" also carries a quiet double meaning, suggesting the dropping of leaves and the dropping of the year. The poem lets that sense hover and still ends on brightness.
What makes the poem worth reading is that it argues for a way of seeing without lecturing. Its subject is an ordinary garden chore, and judging by the imagery, probably burning away what the season has left behind. The speaker treats it as a spectacle. The poem suggests that cheerfulness is partly a matter of where you look. If you stare at the bare flowerbeds, you see an ending, but if you look up the valley, you see a row of red fires and tall grey columns of smoke. Each season has its own beauty, and noticing it is a skill we can practise.
For a reader your age, that is a useful idea with a very small amount of machinery behind it. Twelve short lines, one clean rhyme in every stanza, and two contrasting colours are enough to carry it. The poem's simplicity is deliberate, because a poem about finding the bright side shouldn't be hard work to read.