To begin, we must first understand the poetic conventions of the sonnet.
A sonnet is a verse poem that has 14 lines, 10 syllables per line, and adheres to the following rhyme scheme:
ABAB CDCD EFEF GG
This means that the poem consists of 3 quatrains (three sections with 4 lines) where every other line in each quatrain rhymes. Finally, the sonnet ends with a rhyming couplet.
Elizabethan sonnets are written in iambic pentameter, which refers to the metrical "feet" of the poem. In this case, an iamb is comprised of an unstressed syllable followed by a stressed syllable. Pentameter (consider the root of the word: Penta meaning five) means that there are 5 iambs per line, for a total of 10 syllables.
So now that we understand what type of poem we are composing, we must consider Friar Lawrence as a character. He is an important figure, without whom the play would not have unfolded as it did. Early in the play, he counsels Romeo not to be too hasty:
"These violent delights have violent ends. Which in their triumph die, like fire and gunpowder, which as they kiss, consume." (2.6.9-11).
But in spite of that, he decides to marry Romeo and Juliet in secret in the hopes that it will end the feud. He also gives Juliet the means to feign death, and in that way, is a primary reason for the play's tragic end.
Shakespeare is very clever in the way he writes his characters, and gives Friar Lawrence speeches in the style of sermon and sententiae (moral sayings or maxims). This is something to consider when composing your sonnet: he is a man of God; how might a man of God speak?
Furthermore, you must consider the recurring themes in the Friar's dialogue. He spends a lot of time talking about plants, and indeed it is those plants from which he distills the poison that will make Juliet seem dead.
Within the infant rind of this small flower
Poison hath residence and medicine power.
For this, being smelt, with that part cheers each part;
Being tasted, stays all senses with the heart.
Two such opposèd kings encamp them still,
In man as well as herbs—grace and rude will. (II.iii.23-28)
Think about how he functions in the play: as a man who foreshadows the tragedy
, and is also a part of it; as someone with wonderfully good intentions, who ultimately has a hand in something terrible; as a kind man who only wants to help, but ends up making things worse. He's a very compelling character, and is certainly worth the 14 lines.