How do I describe the structure of the poem?
You could describe the structure of a poem in a variety of ways, but generally you should focus on stanzas, rhyme scheme, rhythm, and meter.
For the poem that you chose, there are 4 stanzas. They are not of equal length. Stanza one is 9 lines long. Stanza two is 10 lines long. Stanzas three and four are 12 lines long.
In general, the poem follows an AABBCCDDEE rhyme scheme. What's cool is that because stanza one is an odd number of lines, the rhyme scheme flows from its last line into stanza two's first line.
Long lost to realms beneath the northern skies (stanza 1)
She shines supreme, while hated faction dies: (stanza 2)
The rhyme scheme doesn't always follow the above pattern exactly. For example, lines 6,7, and 8 in stanza two are all the same rhyme. That fixes the issue of a rhyme flowing over into the next stanza.
As for rhythm, the poem is written in the iambic foot. That means each line follows an unstressed/stressed pattern. Each line has 10 syllables, which means each line has 5 feet. In poetry talk, that means the poem is written in iambic pentameter.