Familiarize yourself with the structure of the Shakespearean sonnet. Shakespearean sonnets consist of three quatrains, or groups of four lines, and a final couplet, or pair of rhymed lines. The rhyme structure is A-B-A-B C-D-C-D E-F-E-F G-G. The letters identify when a word must rhyme with another word. Each new letter indicates a word that doesn't rhyme with one that preceded it. For example, the first line -- indicated with an A -- might end with "dog," so the third line -- also indicated with an A -- must rhyme with "dog." Meanwhile, the second line -- indicated with a B -- cannot rhyme with A, but will rhyme with the fourth line.
Familiarize yourself with the poetic meter of Shakespearean sonnets. Shakespearean lines are written in what is called iambic pentameter. An iamb is a metric foot (i.e. unit) of poetry consisting of an unstressed syllable followed by a stressed one. "Pentameter" means that there are five iambs in each line. A stressed syllable is one in which there is emphasis when you say it, while an unstressed syllable does not have emphasis. For example, in the line "Roses are red," the "Ros" part of "Roses" is stressed, as is the word "red."
Read a lot of English sonnets. Shakespearean sonnets are named after Shakespeare because he is the most famous and most accomplished author of this form of poetry, but he is not the only one worth reading.
Write down your reactions to the sonnets. Think about the meter, rhyme scheme, subject, flow and anything else that catches your eye.
Go back to your notes. Which Shakespearean sonnets captured your interest? You could borrow a theme from one of your favorite poems, take off from where it ended, or even write a reaction to it.
Begin to write your poem. Don't worry about making every line exact. Imperfect rhymes and slight irregularities in the meter are nothing to be worried about. This is a first draft, after all.
Read your poem aloud. This is a crucial step whenever you are writing poetry. Reading out loud to yourself will help you hear what parts sound good and what could use a revision.
Revise your Shakespearean poem. Feel free to use a Thesaurus to help find the perfect word.