Consider Your Subject
When writing an Italian sonnet, it is best to consider, first, a question or an argument. What is more beautiful -- the ocean or the forest? Can a man truly love more than one woman? What is the definition of forgiveness?
In an Italian sonnet, a poet's speaker (or narrator) must first lay out an argument, significant question or observation. However, by the poem's end, the speaker will have had to change his mind in some way, arrived at some new conclusion. Because of this, choose the argument that inspires your poem carefully; stick to ideas about which you, yourself, are perhaps ambivalent.
Organize Your Thoughts
Italian sonnets are divided into two stanzas. First, there is the octave -- a stanza of eight lines. Then, the sestet -- a stanza of six lines.
This division is in no way arbitrary. It is in the first eight lines of the poem that the speaker must argue the key claims behind his ideas. Then, when the poem breaks after the eighth line, a "volta" or "turn" occurs, during which the speaker starts to see things differently. The final sestet stands on its own, answering the first octave as the speaker offers reasons for self-doubt.
Draft the Poem
Before attempting to write your Italian sonnet complete with rhyme and meter, draft it out in plain language. You should still adhere to the 14 lines, matching the argument and its turn accordingly. However, avoid thinking about sound for now.
Too often, when a poet sets out to rhyme, the rhyme scheme ties his hands, causing him to settle for end-words that are "close enough" or downright awkward. Know what you want to say first, and when the time comes to rhyme, reserve only a small amount of wiggle room when choosing your words.
Fit the Content to the Rhyme
Poets sketch patterns of rhymes by “coding” end-words that rhyme, replacing them, instead, with letters. The first set of end-rhymes is marked with "a." The second set is marked with "b," and so on. So, in a poem with the rhyme scheme abab, the first line rhymes with the third line, and the second line rhymes with the fourth line.
The rhyme scheme for the Italian sonnet's octave is ababcdcd; its sestet is cdecde. Knowing this, examine your poem as is and see which end-words can be changed or replaced with synonyms in order to make the appropriate lines rhyme. Sometimes, you might have to tinker with syntax to make for better rhymes; sometimes you might have to rearrange your thoughts entirely in order to stack end-words that suit the rhyme scheme.
Refuse to Be Overwhelmed
Making your text suit the Italian sonnet's rhyme scheme will likely take a great deal of time. Don't worry; the challenges offered by following a rhyme scheme will likely cause you to problem-solve in creative ways and might even allow you to produce lines and phrases that wouldn't exist if you hadn't been pushed.
Allow yourself to play with a number of rhyming variations before making any final decisions. For this stage, it's best to write your sonnet on a computer so that words and even entire lines can be deleted and un-deleted with simple keystrokes.
Choose Your Meter
Originally, Italian sonnets were organized to show off the sounds of the Italian language. However, compared to Italian, English has far fewer words that rhyme, and far fewer words with multiple, complex arrangements of stressed and unstressed syllables. Because of this, when the Italian sonnet started to gain popularity with English poets, many rules regarding rhyme and meter started to change so the poems could become easier to write. Most English poets settled on iambic pentameter, a pattern in which an unstressed syllable is followed by a stressed syllable and repeated five times in a single line.
Contemporary poets are given even more freedom when they attempt the Italian sonnet. If you like, you can abandon meter altogether. However, if you insist upon doing things the old-fashioned way, it is a matter of convention that you stick with iambic pentameter.