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Think of an inspiration.
And then you can plan for the poem, ask these questions :
Is there a syllable-length? If yes, what is the length per line? 
Example: You want a poem where there should be 10 syllables per line.
And then after you constructed your first line, look for a word that will match the last word on your first line, you can surf the internet for this. And then continue on doing your poem. 
First think of a good topic .
jolt down everything you know about it.
make short sentence ending with rhyming words.
after doing so, syllabicate it then edit.