The content of the sonnet will depend on which part of act 2 you choose to write about. For example, a sonnet from Lysander to Hermia or Hermia to Lysander will be different in the earlier part of act 2, before Puck puts the love potion in Lysander's eyes that causes him to fall in love with Helena. A good place to put a sonnet from Lysander to Hermia might be right after the point that Hermia insists that they sleep a little apart. He could be watching her from a distance as she sleeps in a beam of moonlight with the flowers around her lit up as well, and compare her beauty to the beauty of the forest. He finds it beautiful, but she is more beautiful still.

We know Hermia is shorter than Helena and has a fiery personality, so both these aspects could be put into the sonnet. He could in the third stanza write of longing—longing to be with her, right by her side, not modestly apart, and longing for tomorrow. The final couplet of a sonnet usually has a twist, so you might write about him both being so grateful to have escaped Athens with his beloved mixed with ingratitude at the sun for not rising sooner so they can marry.

Remember that a Shakespearean sonnet consists of three stanzas of four lines each and then a rhyming couplet at the end. You might imagine yourself the lovesick Lysander and describe Hermia's sleeping beauty amid the purple forest flowers in the first stanza, talk about how she is more beautiful than even this beautiful forest in the second stanza, speak of your longing in the third stanza, and describe how your love will never, ever change (ironic, as it soon will), and then end with the final couplet.

Remember too that the stanzas each follow their own ABAB rhyme scheme with the first and third and second and fourth lines rhyming, and are in iambic pentameter, five two-beat syllables of da DUM, da DUM.

In Act II of A Midsummer Night's Dream, Helena is pursuing the disdainful Demetrius and becoming ever more abject in her professions of devotion:

Use me but as your spaniel, spurn me, strike me,
Neglect me, lose me; only give me leave,
Unworthy as I am, to follow you.

A sonnet might well use the same or similar images as those we actually hear from Helena in the play, where she asks Demetrius for leave to follow him around as his dog does. Here is an example of an octave in which she speaks in the same vein. I make no great claims for this as poetry, but it follows the rules of the Shakespearean sonnet and conveys the meaning well enough:

I do not ask for kisses and kind words.
Only your presence and proximity.
This is not much, Demetrius. The birds
And insects of the air may follow thee.
Do not regard me any more than such
Small creatures which you see but think not of.
Though I am near thee, close enough to touch,
I shall forbear, but bask in mine own love.

To recap briefly on the rules for a Shakespearean sonnet, it is a fourteen-line poem written in iambic pentameter with the rhyme scheme ABABCDCDEFEFGG. I have composed an example of the octave (first eight lines) here. There is normally a turn, a change in meaning or emphasis, at the final couplet. You might, for instance, have Helena make the point that, although she will accept the most discourteous and indifferent treatment from Demetrius, she has done nothing to deserve it.

Yet though I live to look upon thy face,
My love deserves more courtesy, more grace.

There are, of course, many other ways of ending the sonnet with a contrast. You might, for instance, compare Demetrius's ungenerous conduct with that of Lysander to Hermia.