Master the meter. Sonnets are written in iambic pentameter. Pentameter means that there are five meter units per line. Iambic means that each meter unit is an "iamb," in other words, two syllables with the emphasis on the second syllable. For instance, the word "report" is an iamb, while the word "butter" is not.
Choose your rhyme scheme. A rhyme scheme is the rhyming pattern for the words ending each line of the sonnet. An Italian sonnet uses an "ABBA/ABBA/CDE/CDE" pattern. This means the first, fourth, fifth, and eighth lines rhyme (the "A" in the pattern), while the second, third, sixth and seventh lines all share a different rhyme. A Shakespearean English sonnet uses the "ABAB/CDCD/EFEF/GG" pattern. The final "GG" lines are called a couplet, and typically provide a more potent ending than the Italian form. Over the years, different poets have departed slightly from these patterns, so feel free to innovate.
Carefully plan your structure. In addition to simply dictating the sounds of the final words of these lines, the rhyme scheme imposes a certain logical structure. The Italian sonnet has four basic sections, while the English has three, followed by the ending couplet. Use these sections to the sonnet's benefit. Just as a narrative has a beginning, middle, and end, so can a sonnet.
Begin writing. Once you know what you are trying to say in your sonnet, finding the right words for the meter and rhyme scheme should be easier.
Edit your sonnet. As with any other piece of writing, a first draft is rarely the best draft. Go back and correct word choice, move lines around, or fix a problem with meter. Read your sonnet aloud each time you complete a revision. Difficult-to-edit portions will stand out better when spoken.