Choosing a Form
Since the sonnet is a relatively strict type of poem to write, you should first decide what kind of sonnet you want to write. The Petrarchan sonnet follows a rhyme scheme of ABBAABBA -- where all "A" lines and all "B" lines rhyme with each other -- for the octave and CDECDE or CDCDCD for the sestet, where matching letters stand for rhymes at the last word in each line. A Shakespearean sonnet uses the rhyme scheme ABAB CDCD EFEF GG. The Spenserian sonnet uses quatrains and a couplet, but it follows the scheme ABAB BCBC CDCD EE. You can even choose to make up your own rhyme scheme while maintaining a 14-line poem.
Getting the Idea
Consider the subject matter of your poem and stick to one concept. Many traditional sonnets are about love and nature; you can follow this traditional approach or write about any idea you have. Sonnets also tend to state a problem, question, argument or observation, and the latter half or end of the poem brings clarification or a counterargument to the initial statement.
Writing Iambic Pentameter
Using iambic pentameter without it sounding forced can be difficult. Read some sonnets first to get an idea of the meter and the musical nature of the rhythm. To start, write what comes out naturally, focusing on the rhyme scheme. Edit the lines later to put them in iambic pentameter. Consider monosyllabic words, which can be accented or unaccented, depending on their position in the phrase. You can break the meter, but meter breaks should relate to some important event or idea in the sonnet; this technique gives attention to that point in the sonnet.
Focusing on the Turn
The most important part of a sonnet is the turn, or volta. The turn answers the question, provides a counterargument, summarizes the rest of the poem or brings a surprise or clarification to the poem's opening idea. In Shakespeare’s sonnet “My Mistress’ Eyes,” the first 12 lines express how the speaker’s mistress does not look like the beautiful images he describes, but the final two lines admit his love is rare and incomparable. The Shakespearean sonnet places the turn in the final couplet, while the Petrarchan sonnet puts it around lines eight or nine, near the start of the sestet. Analyze some sonnets to get an idea of where to place the turn.
Revising Your Work
Even if you work with the structure on your first draft, revision is key to making your poem the best it can be. Read the poem aloud and focus on the rhythm and meter. Fix any places that aren’t in iambic pentameter. Find lines that could use enjambment -- a technique that carries the phrase to the next line to avoid all end-stopped lines. Check your work for concrete imagery, and avoid generalizations or overly abstract thoughts.