Master the art of iambic pentameter. Some contemporary sonnets are written without its use, but you will want a good grasp of the classic forms starting out. This means understanding meter, or the way syllables and stresses are arranged in a line of poetry. The patterns stressed and unstressed syllables form are called "feet," and the iamb is one type of foot, consisting of an unstressed followed by a stressed syllable, much like the da-DUM of your heartbeat. A line of iambic pentameter consists of five iambs, as in this famous line from Shakespeare's Sonnet 130: " my MIS / tress' EYES / are NOTH / ing LIKE / the SUN."
Select a sonnet form: Italian/Petrarchan, or English/Shakespearean. Many find the English form simpler to write, so if this is your first try at the sonnet form, you might start there. The English sonnet consists of three quatrains of varying rhyme followed by a couplet: abab cdcd efef gg. The rhyme scheme of the Italian sonnet is more difficult to sustain. It contains one octave, or eight-line stanza -- abbaabba -- followed by a sestet: cdcdcd. There is also a "Spenserian" variation, and many sonnets that mix the established forms up a bit.
Choose your subject. A sonnet is generally structured as either the poet's meditation on a subject or an address to a loved one. The classic subjects for sonnets are predictable: love, death, nature, the passing of time. Making such topics -- or any topic -- your own means being very specific and concrete. Not just love, but a particular beloved; not just nature, but a single scene or moment. You might also push against the classic form and pick a decidedly non-poetic subject for your sonnet -- something funny, or personal, or unexpected.
Understand the volta, or turn. This is an essential characteristic of the sonnet form. The turn represents a shift in perspective, a surprise, or a change in the poet's line of thinking. In the Italian sonnet, the turn occurs between lines 8 and 9. In the English sonnet, the turn generally appears between either lines 8 and 9, or before the final couplet. One example of a strong turn is in Shakespeare's sonnet 130; after 12 lines describing his mistress' less-than-ideal physique, the final couplet reads: "And yet, by heaven, I think my love as rare / As any she belied with false compare."