Read the poem out loud. Listen for syllables that you say louder and syllables that you say more quietly. It's OK if you can't tell them apart yet.
Open a dictionary. Look up each two-syllable word in the first and second lines of the poem.
Use the dictionary to find where the stress marks are placed. You can find the stress marks in the pronunciation guide immediately following the word itself. Stress marks usually look like a single quote (') and appear before the syllable that is stressed.
Mark, on the poem itself, a small line above each syllable you know is stressed. This line should look like a forward-slash.
Practice saying these words so that you over-emphasize the stressed syllable.
Mark all single nouns and verbs with a stress mark. These marks may change later, but there's a higher chance that these single syllables will be stressed while articles (a, an, the), conjunctions (and, but, or), prepositions (in, of, from) and other minor words will not.
Mark above all unstressed syllables with a u-shaped curve.
Read the first and second lines again, this time over-stressing the syllables where the stress marks are, and under-stressing where the unstress marks are.
Repeat the process from Step 2 with more lines of the poem until you feel confident that you can figure out the stress simply be reading the poem aloud to yourself.