To annotate a text is the same thing as glossing a text. To gloss (annotate) a text means to write a detailed commentary of a text. A commentary of this sort will be somewhat different from a critical analysis although some of the same topics may be covered.
Holmes opens his cheerful and light-hearted poem by modifying the expression "old times," meaning times of life gone by, to match and parallel another expression, "the old friend," which appears later on in the poem. So the focus of the poem is on the parallel ideas of "the old time" and "the old friend."
Holmes then employs a poetic convention by speaking of "the buds of April" blossoming, which symbolizes the spring time of life, or youth, while literally representing "the old time" that Holmes is focusing on. A second poetic convention of "birds of spring-time" singing symbolizes the happiness of youth and also represents "the old time." It is probable that "gardens brightest glories / by summer suns" nursed is another convention that symbolizes growing to maturity in a garden of life and friendship while also again representing "the old time" in the literal garden from his recollection.
Holmes now leaves the garden and switches to the other half of the parallel focus, "the old friend." The phrase "morning days" operates the same as the phrases above: it symbolizes days of youth while representing literal mornings, probably spent in the garden of the first part of the poem. Holmes again switches and begins a long metaphor comparing friendship to fame. He says a friend's "welcome" and "homage" have no equal.
A colon follows "like his praise." The colon indicates that what comes after explains or expands upon what comes before the colon. In other words, because of the colon, we know that the last lines are the continuation of Holmes' thoughts about friends and not a new topic altogether.
The last five lines might be paraphrased like this: There is no homage (honor) like a friend's praise: This is because fame is like a scentless sunflower with no real sweet charm of its own and only the appearance of gold in its circlet of petals; but friendship is the real flower in the metaphoric garden of life--it is the sweet-scented rose that has fragrance in every petal. Holmes uses similes to compare fame to "the scentless sunflower" and friendship to "the breathing rose," while "breathing" also implies life-force and represents a personification of the rose.