When looking at poetry, do remember that the message is immersed in rhythm, meter, sometimes rhyme, and unconventional wording and punctuation. In order to "rewrite" it, you must first dig out the complete ideas. Too often, students stop at the end of a line when reading, even if there is no punctuation to indicate stopping. Concentrate on reading line--wrapping around to the next if necessary--until you reach a period, semicolon, question mark or exclamation mark. Then, take that one idea (line) and write it in prose. Sometimes you will need to read the entire poem a few times to be able to do this since the lines work with each other for their meaning...they are not independent of one another.
With any poetry, but especially sonnets, you must read the entire poem a minimum of three times to get the meaning. First for general content, then for changes in tone or mood, and lastly for the rhythm and beauty of the language. Sonnets almost always present an issue or problem in the first section of the poem, then change tone or mood with a conjunction/transition (I like to call this the big BUT), and then finish with a solution to the problem or a final comment on the situation.
Sonnet 29, for instance: When I am depressed and no one likes me and I want to be what everyone else is, THEN I think of you and am so glad to have you that no matter what is going on in my life, I would not change places with kings.
An intriguing question. How you would rewrite sonnets would depend on what your goal was in doing so. If you are trying to write your own poem in response, your goals would be much more open-ended. Howard Moss produced a fair amount of irony and humor in his rewrite of one of Shakespeare's sonnets (see the link below).
If you're writing to paraphrase them into prose for other purposes, you'd look at the literal meaning, but also at how the sonnet structure affects the meaning and impact of the poem. You'd look at each of the quatrains and how they relate, as well as the sense of progression. You'd also look at the final two lines, to see how they reflect or sum up the lines before