Decide on your topic. Brainstorm ideas, think of different problems or questions that you would like to deal with and possibly provide an answer in the end. Your poem's main theme does not have to be complicated or deep, it can be something simple, as long as you feel strongly about it and as long as you have a lot to say on the subject.
Familiarize yourself with this rhyme scheme. The octave's most common rhyme pattern is "abba abba," which means that the first line rhymes with the fourth, and the second with the third. Milton's sonnet 19 is a good example:
"When I consider how my light is spent
Ere half my days in this dark world and wide,
And that one talent which is death to hide
Lodg'd with me useless, though my soul more bent"
Write and rewrite your poem. Keep writing, reading and making adjustments to your poem in octave. Do not just settle for a first draft; work on it for a while, pay attention to the details, rewrite the whole thing if you have to. Remember that writing is rewriting and no poet ever delivers a masterpiece in his first attempt. Once you are completely satisfied and you can see no more room for improvement, then it is time for some editing and proofreading. Do your last revision and scan your poem for any spelling or grammatical errors, but at the same time remember to enjoy the freedom with words that poetry offers you. Come up with a strong title for your poem.